I understand what you're trying to say...if the rubber mats dont work I would try just placing some shims under whichever corner needs to be balanced. I did this with my 29gal when I had a similar problem. The shims have held up quite well and my tank sit's smoothly on the shelf and its been over a year now. I think I spent maybe $2 on a pack of shims and problem solved!
Hope that works for ya.
Hope that works for ya.
