What about just like some 3/4" or 7/8" plywood, you could get a big piece and cut it to the rigjt length and width, it would be low enough but still provide even support. Then you could just pound shims in around where necessary.
I was always afraid to shim tanks directly, I had a bad stand once and I came close to shimming underneath the rim but decided it might put too much pressure or stress in that spot. I guess I'm kind of speculating here, you very well may be fine without the plywood
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