Steam box - the smaller the better. All I used is a piece of aluminum leader (gutters and leaders) with a wood stop on one end. I drilled a few small holes so I could insert some wire shelves. It was as long as my longest rib. With bricks, I suspended it over an electric tea kettle that I soldered a tiny piece of copper plumbing pipe into the lid which shot steam up into a hole in the center of the leader. I put my ribs in the cooker and just shoved a rag in on that end. 2-3 minutes, white oak like rubber.

My point is if you have some leader around, it works fine with no insulation etc. just shoot steam in from under the middle.
If you do make one, make it about 3" square with some wire inside for shelf. You don't have to make this giant steam box.

To bend small stock for coamings and lips, I used to use 1" plumbing copper pipe with an elbow soldered on the end that went over the steam source. Needle nose pliers to pull it out. That I did on my kitchen stove with a steam cooker. I would carry the entire pipe down stairs and take the wood out just before using it.