@aviator8 Did you find a solution?
For what it’s worth........ How about adding a strap around the tank as high as you can get it. Attached to that strap would be your weight or some kind of small weight pocket which would be easy to sew up. To keep the strap from slipping, I’d suggest putting dots of silicon caulk (or silimlar) on the inside of the belt.
I did, I got a few 1 lb'rs and plasti dipped them