There is always this though it sounds like you might want a car inner tube to cut rubber bands for it.
As for gluing; if you fit the two edges together you have a ziz-zag line that is the cut and that is also all the faces that will be glued to the edges and bend that 30 degrees or so and you expose all those edges as well. A very light bead of glue down the center of that gutter and you have just put the glue on every face to be glued. A bit of light wiggling and it will be on every needed face. I keep a wet cloth and some q-tips to wipe away any extra.
Every time you make a box with fingers, you also make a bit of scrap with fingers that you can glue together to make some angle with fingers and experiment making those first and you can use those to hold the glued corners as well.