Hi, How are ya? I just noticed the message, sorry for the delay.
I went with 48" Wide x 24" high x 5' long. I tried to account for all things I knew I wanted inside, like storage totes. The frig, with the rack I built for it just fits perfectly, with the 1/2 spacer under so it did not scrape...
I got form Hayneedle. I would say no dust, got rained on, no water. I did add 1/2 in plywood liner to it, all around, except the lid.
http://www.hayneedle.com/product/tradesman16intrailertonguebox.cfm
Gas struts came from McMaster Carr .... they have everything. Fast shipping to.
Build is here http://tventuring.com/trailerforum/thread-519.html
Struts:
http://www.mcmaster.com/#gas-springs/=toyjzr
put gas struts, 50 # in each upright, got lucky, up & down easy with 2 people or set latch pins & get on top, easy down & lock. Tailgate slides off back & into 3/4 sq welded to the uprights, ratchet strap to level the table.