There are some good points in favor of a cargo trailer build-out. Choose between aluminum and steel. CM Trailers' Cargo Mate is an all-steel trailer that comes to mind; a galvanneal steel body has no riveted/screwed seams, it's all welded. The roof is rolled over at the edges so there's no sharp corner seam to leak. I've sometimes thought how nifty it would be to have a trailer with a ramp door that could be dropped down to jacks so it stays horizontal, and acts as a sort of porch or patio. Put screen material across the door opening and have a monster 'window' for enjoying the outdoors while inside, too. The square shape would be easier to build inside than an rounded egg, but of course you'd give up some fuel economy and all the
fiberglass chic (let's face it, with a cargo conversion you won't have people lined up at the CG to see your "cute little trailer"!).