I was able to calculate the dimensions using a little photogammetry as 6.5" tall, 2" deep, and 2.75" wide (just right if you're looking to carry a whack of pencils and pens I guess). I didn't have any old couches on hand (and these days upholstery's all artificial leather anyway) so I went with stone oiled leather. To get the rigidity and sharp corners I made what amounts to a latigo box or lining and then wrapped the pouch around that. With the outer layer holding all the latigo in place the whole box is surprisingly sturdy. So much so that I'd compare it to 1/4" plywood or plexiglass, except that this won't risk cracking or splintering.
A note on the photos: As is increasingly the case, I didn't have time to wait for the perfect outdoor conditions before shipping the order. In this case it was a particularly overcast day and all the diffuse natural light is making the matte black leather look more silver or grey.