Not affiliated or benefiting from anything to do with this, but in the Jacksonville, Florida Craig's List there's an interesting cargo trailer with conversion possibilites:
New Craig's List link:
http://jacksonville.craigslist.org/tls/1199968410.html
I'm leaving the first ad quote below since the second ad with the lowered price is a little different (see a few posts down). I know sometimes there's a story to be read between the lines when an item is reposted with a lower price and different ad wording. It's not always negative or positive, but there's usually something to be read there!
THIS IS A 4X8 ENCLOSED TRAILER
aluminium and steel frame .
rear door lifts and has small side door
well made traler that tows great no complaints myself
new 3000 pound cuopler from faslock
recenly painted grey and black
wood roof racks will come off easy
new leaf springs professionally installed by What Now Trailers
i have had atleast 2000 lbs of concrete in it and it towed fine.
shelf in front with lip
1\8 inch corragated alluminium on front triangle for extra cargo has d hooks to secure as well
decent tires

I particularly like the tongue platform that's already on it and that the tailgate lifts up instead of being a ramp (see the other photos in the Craig's List link -- that more than anything make it stick out for good conversion possibilities). It's also a rather small trailer to have a side door....usually the ones with side doors are bigger. The trailer does look a little rough, but seems to have good bones and a nice axle/suspension. It does seem a little "butt heavy", but that's the way most cargos are. At that price, I'm more than a little tempted myself, but I promised myself to only consider 5' wides (both tears and cargo conversions) as that's what I need for camping with three people. I figured someone else might be able to take advantage of it.
I'm close enough that if someone wants me to go take a visual, in-person I will -- take more pictures, etc. Just PM me. I cannot guarantee that if I go see it and like it that I won't buy it myself, tho!