[JulianEdgar]

You do not want that sharp front edge! With it, it is quite likely that flow will be separated down the side of the trailer, increasing drag.

The sharp edge needs to have a radius. I don't think the radius needs to be all that large (50mm?) but without it, drag is very likely to be much higher than it would otherwise be.

If you want to avoid my guesswork (always a good thing) tuft test the front and sides of the trailer as it is now, before doing any further work.

Addition:

I've never really thought much about trailers, but if you're happy with fabrication, you could make a temporary coupling with a spring within it, that extends with the aero and rolling resistance of the trailer. (Obviously add a safety device so that if the spring fails, the coupling doesn't come apart.) Add a pointer and scale and attach a video camera or similar. Then on a flat road at a constant speed, you could directly measure changes in aerodynamic drag in real conditions.

Further addition

You may well have your own ideas about how to form such a radius but I'd probably use thin wall steel or aluminium tube and roll it to the curve, or uPVC plastic pipe and heat/bend it to shape. In either case, I'd then rivet or glue the panels to the tube, so forming the radius. (Some fibreglass cloth and resin could then cover any 'steps' that developed.)

Or, if you have a bead roller (my new toy, thanks to prompting from Freebeard), you could roll the radius'd curve in aluminum or steel using a urethane lower roller and curved upper roller.