I did this and it works great. For the floor I attached 3/4" PT plywood with self tappers to the frame, and used two pieces of perforated angle and midpoints in between. Then I made the walls out of 1/2" PT plywood and 1x4 PT frames. I installed cut-outs at each corner to accept the extended 1x4 vertical frame pieces, added some hasps and slide bolts at the tops of each corner and voila, one utility trailer comin' right up.
The best part is being able to remove the walls so easily, and, when in use they are very affective. I've had numerous compliments from neighbors and sailors. So far I've hauled all kinds of different stuff from mulch to mattresses and bed frames to lumber and kayaks, etc., etc., knowing that I've been very close to the 700lbs. weight limit of the tires. Let me know if you are interested in pictures.
The only difficulty I've found is managing sail and boom storage and transportation. I've struggled with that, first going with a dual 4" PVC scheme where I mounted them under the bed and above the axle in which I would put the boom and battens. That does work, but I decided that I don't want to hassle with dismantling and reassembling my sails and boom and battens and then having to fold and bag my sails each and every time.
The only other options were to go with a large diameter tube, buying a toy box, or building a box. Building a box would be fun but I've seen enough that just don't hold up well over time, plus removing it, storing it and installing it when needed would be an undertaking alone, and I want something I know will last. Plunking down the moolah for a toy box seemed silly and I'd have a similar issue with handling it and storing it. The cost for large diameter PVC and fittings was a little high, too.
So, just this week I ran across a reasonably priced 12" x 20' piece of corrugated, double-walled, black plastic culvert pipe on craigslist and I jumped on it. I plan to install recessed anchor points in the floor of the trailer that will pop up when needed for the tubes and fall back down when in utility mode. To these I will use ratcheting tiedowns to secure the tubes to the bed. The culvert is perfect, it's light enough to handle myself and tough enough to last a lifetime. I'll use one 10' piece for my sail and boom rolled up and in its bag, and the other for rudder assembly, hot stick, and any other random piece of gear that needs a home.
The only thing I haven't resolved is whether or not I want to deal with ~3' of the tube hanging off the back end. With the boat loaded the transoms come to approximately the same distance from the back of the trailer, but, I may end up reconfiguring my two tool boxes mounted in front of the trailer bed. One is lengthwise fore and aft, the other is lengthwise side to side. Switching the former 90 degrees and moving the latter towards the front of the trailer will give me almost all the distance I need, less say ~1'.
For what it's worth....
If it ain't a blowin', I ain't a goin'