I built a portable darkroom box last year, for changing film holders and processing 4x5 negatives. It's a box made from thin wood paneling attached to a space frame of 3/4" spruce sticks. The top is removable, has a light trap made with a groove and flange method. Insides painted black. Handles on the side for carrying. The key feature is arm sleeves, made from the black oilcloth tablecloth material, available at fabric stores. The black vinyl material is on the inside, the white cottony lining is on the outsides. I used a hot glue gun and gaffers tape to make the sleeves. They are attached to the front of the box with a flange gasket made from thin wood. The outside of the box is finishing in varnish.
This design is small enough to carry in a car, but too large to carry anywhere off the beaten path. You could perhaps use the idea of arm sleeves, in a more portable or lightweight box. I think the idea of a heavy but light-tight fabric, seamed together with hot glue gun and gaffers tape, is doable by almost anyone with meager mechanical skills; you don't even need to know how to sew or have a sewing machine.
Here's a picture of the developing box, on a portable camping table at my local city park, where I was shooting and developing paper negatives from my Speed Graphic.
Here's the thread on F295 about the entire project, including construction details that may be of help to you:
http://www.f295.org/Pinholeforum/forum/Blah.pl?m-1230847394/s-all/
Good luck, keep us informed of the progress you make.
~Joe
PS: This box has come in very handy during the winter months when my garage-based darkroom is frigid cold. I still preflash, cut and load paper negatives in the darkroom, but the portable box is a convenient way to process the negatives in the comfort of my kitchen.