Hi Jinky, I bought a trailer for wfp from these guys,
http://www.indespension.co.uk/ , i went to their shop in belfast and got a Daxara 148 open trailer, they were really helpful, they build the trailer you want from scratch, with all the features that you want.
I didnt kit it out with ply as it's an open trailer i didnt think the ply would stick the weather too long. I just built a heavy wood frame to keep my two 175L upright tanks apart from each other and it keeps them pressed up against the sides of the trailer and i made another frame to keep them pressed up against the front of the trailer. I just use a 3ton strap to prevent the tanks from jumping out and to keep the frame in place which i have attached my pump/di/controller box.
I wouldnt worry too much about having to put a few holes in the side or base of your trailer, you can seal the holes up with a dab of silicone, plus you may need a couple of drainage holes as you are dealing with water!!
I know that Indespension does roof racks for their trailers so you can mount your poles outside if you cant fit them inside. if the poles are just slightly longer than your trailer, why not try putting them in on the diagonal, you might just be able to fit them inside then.
Hope this helps,
Tim