After some experimenting, this is the best way we’ve found to add an extended flatbed to a Harbor Freight trailer to make a functional fly-on/fly-off hovercraft trailer. We begin with an assembled Harbor Freight 1195 lb capacity heavy duty folding utility trailer (Item #90154). You can usually get these on sale for around $300. Follow the instructions to assemble the trailer, with the exception of leaving off the hinge parts that allow the trailer to fold when not in use. Once the deck is added, the trailer will not fold, so these parts are not necessary. Also leave off the casters (only used when the trailer is folded), as well as the tail lights and license plate holder (until later).

This trailer is 48″ x 96″, so the first step is to add rails along the long dimension to extend the trailer forward and backward. The bed in this example is about 7 ft x 13.5 ft, which is a little longer than we normally recommend for balance reasons.
To accommodate a small hovercraft, you can easily extend 2×4 rails forward ~16″ and backward ~20″ from the normal bed. Using these dimensions will result in an 11 ft long bed, which is perfect for a UH-10F or similar small craft. It is important that these 2×4 rails be secured to the metal frame. The Harbor Freight trailer has 3 pre-drilled holes that can be used for each rail. We use 3/8″ bolts with thread locking compound to secure the 2x4s to the frame.
The rails extend the bed, and 2×4 cross members laid on their side across the rails and metal frame will actually support the plywood bed. These 2×4 cross members are laid down every 2 feet and can extend as far out to the side as you need. In this example, the cross members overhang the trailer by about 16″, resulting in a bed width of 80″. For a smaller hovercraft like the UH-10F, a 72″ bed width is more than enough to accommodate the hovercraft and keep the total weight down. The cross members are bolted to the metal frame wherever the frame has pre-drilled holes (see diagram at bottom of page for detail) and screwed to the 2×4 rails wherever they cross.
In addition to the 2×4 cross members laid on their sides, additional short lengths of 2x4s are also laid on top of the rails in between the cross members. The pieces over the trailer frame are bolted to the trailer using the pre-drilled holes in the metal frame, and deck screws are also used to fix these pieces to the 2×4 rails.
Finally, long 2×4 outer rails are deck screwed to the outer edges of the cross members. This provides additional longitudinal rigidity and provides a convenient location to mount handles, etc.
A view from the rear before decking
A closeup showing detail of cross members mounted both directly to the trailer frame with bolts and to the 2×4 rails with deck screws. Note that the vertical bolts securing the cross members to the trailer frame are countersunk, allowing the plywood deck to sit flush on top. There is a ~1/8″ gap between the 2×4 cross members and the metal trailer cross members. To avoid warping the 2×4 cross members when bolting them down to the trailer frame, we drill holes in 1/8″ plywood shims, and bolt these between the 2×4 cross members and trailer cross members before tightening everything up.
View from the front before decking.
This model required about 3 sheets of 1/2″ plywood for the deck. The plywood was cut so that seams in the plywood were all over supporting 2×4 elements. We use all untreated lumber and plywood for our trailers, as they mostly live indoors, where they stay dry. It also keeps the cost and weight down.
A closeup showing the plywood attached to the 2×4 support structure using deck screws. We like using 1/2″ plywood for decks, but 3/4″ plywood can also be used for a little more strength between the support members. The HF trailer is perfect as a base for most small hovercraft because the rails are 4 feet apart, which is very close to the separation of the landing skids for most small hovers. This means that the weight of the hovercraft is supported by the structural rails, and not the plywood.
A coat of outdoor latex paint finishes the job. They’re not shown here, but we also sink eye screws through the plywood deck and into the support frame at the corners to provide tie down points. And the tail lights and license plate holder are just screwed into the sides of the rear 2×4 extension rails near the back of the craft.

The following diagram (click for large version) shows details of where the 2×4 frame is anchored to the metal trailer frame using 3/8″ bolts and thread locking compound. All vertical bolts are countersunk to allow space for the plywood deck to sit flush on top of the support structure. The red dots show locations of deck screws fixing the deck support structure to the underlying 2×4 extension rails.

IMPORTANT NOTE on balance and towing: Vehicles require weight on the trailer tongue to tow a trailer safely. In this particular example, we extended the bed backward further than normal (3 feet) to accommodate a 12.5 foot hovercraft. We load the hovercraft backwards to get enough weight in front of the axle for safe towing. For shorter hovercraft, where the rear of the bed does not have to be extended as far, forward-facing loading is possible with an extended trailer like the one shown here.

With a folding jackstand added to the tongue, a small trailer such as this one can easily be moved by a single person. Since no parts of the bed extension or bed are bolted to the tongue, the trailer also retains its tilt functionality, so removing the two pins on the underside of the trailer allows the entire deck to tilt backward to the ground while the trailer is still hitched to your vehicle.

We’ve had great luck with these trailer kits so far. With minimal lumber and effort, you can quickly build a functional flat bed trailer to haul your small hovercraft. And the obligatory disclaimer: These instructions are provided as a guide only and come with no guarantees. Modification of a trailer kit is done at your own risk.