Hi John. It is a Carry-On 5 x 8 trailer from Northern Tool. Basically it was a frame made from angle iron and did originally have fenders. I decided to remove the fenders for fear that gunk (dirt, mud, whatever) would get between them and the fender boxes, absorb moisture, and cause rot.
This isn't my picture, but here is what it looked like as purchased after the fenders were removed:
This is about the oldest picture I have. It was taken prior to me cutting the fenders off:
During constuction I did reinforce the second crossmember with a second piece of angle iron bolted on with grade-8 bolts. It is imperitive that the second crossmember be bolted to the frame or floor you lay on top of it; otherwise it will bend downwards. Additional reinforcements could be welded to the frame before you start construction--something to consider.
TheDuke, the plastic opening is an access door from the outside to underneath the ledge where the toilet sits. I wanted to reclaim the space for storage and also be able to check for leaks, since any water seeping into that area would potentially rot the wood underneath. I am using a cassette toilet, but it is self contained.