My thinking is that with the existing tie-downs on the trailer that you don't need any new holes at all.
My granddad built a utility trailer out of 2.5t side boards and some DC3 tires (now long gone). When the big move from SoCA to Central OR came along he built a cap for the trailer out of 1by clear Fir and 1/4" plywood, and then glassed the whole thing for water-tightness & structural rigidity. It has a 'register' step on the mating lip such that it fits down over the rails of the trailer. 4 tie straps that pick up the similar lashing points on the side boards has the cap secure to any movement.
I used to swerve around my hallucinations, now I drive right through them.