You could make perfect rounded edges by using hot sand to bend PVC pipe.
Choose a radius you want then select PVC pipe that matches or is close to it. To construct the forming jig I'd cut a 1" long piece of the pipe. On the inside of the trailer mark it every foot or closer. Use the 1" long piece held up into the corner and construct frames to hold it in position at each mark.
Now you have a set of frames to be used much like they're made for constructing a wood strip boat, except here the only concern is forming the chines. Mount the frames to some lengthwise boards so they're held to the same spacing as the marks inside the trailer.
Lay the hot sand filled pipes into the upper corners of the frames and let it cool.
Cut the sides and top of the trailer skin on the line where they were tangent to the 1" long piece.
Cut the pipes lengthwise along that same line for a perfectly fitting rounded corner.
Figuring out how to connect the metal to PVC is up to you.
Covers on the outside of the wheels, covers on the insides too, with as close fitting a hole as you can make around the spindle. Get even more drag cutting by splitting a length of PVC and attach the halves to the front and rear of the square axle.
On the fenders, make new ones or get some plain rounded ones to start with. Mount as low and close as possible to the tires, with consideration given for clearance when the trailer is loaded. Use Cardboard Aided Design to make a cutting pattern for sheet metal to make pieces that taper from the outer edge of the fenders to the sides of the trailer.
Of course all this kind of stuff can add weight which could negate the savings from being aerodynamically slicker. Learning how to do fiberglass is fun but can be messy and smelly. Glassing the cardboard fender mockups then smoothing them to make plugs to make molds to make the finished lightweight fender fairings will take plenty of time but in the end there's a better, stronger, lighter weight product.
Some things definitely needed for a good fiberglass job. TR-104 Hi-Temp Mold Release Wax. PVA mold release (sometimes called "green soap"). Gelcoat resin. A detail spray gun. The detail gun can be used to spay on the PVA and to spray on acetone thinned gelcoat and first coats of polyester resin. Need to clean the spray gun out with acetone before the resin starts to set. Also pick up a copy of The Fiberglass Repair and Construction Handbook, 2nd Edition, published by TAB Books. Old book but 100% applicable today for working with glass fiber and polyester resin.
I would have just put a big vertically rounded front across the trailer with a rounded over top edge, a flat top and a sharp trailing edge, and fitting round fenders as close as practical to the tires. Less work and likely within a hair in efficiency of doing all the extreme shaping for ultimate drag reduction.
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A stretched material can still vibrate. I think that's the problem. Maybe a flexible membrane could be structured to absorb turbulence in the boundary layer. I dunno.
