1- Aspen bedding is fine but you do not need larger than a 20gal tank for a full sized adult. BPs like confined spaces and could actually get stressed from too large a setup.
2- Towels do nothing to help humidity and those open screens will not allow you to keep it anywhere close to where you need it. Use foil and duct tape and seal off most of the top of your tank like so:
Mine holds humidity up in the high 50s/low 60s for days without misting. I also have 2 smallish pieces of terrarium moss in there which does wonders.
3- You can build a small tub setup for 2 BPs for pretty cheap... Take a look in the DIY section for tips. A sheet of melamine, a few sterlites and some flexwatt and you're done. You likely already have the tstat and hides and everything since you've had them for a long time(i sure hope anyway...) so you don't need to re-buy that stuff. With what you already have I can't imagine it costing you more than $100.