Mass-loading subwoofer

I have a Rel G1 MK2 subwoofer (108#) that sits on an Auralex Great Gramma V2 platform (300# limit) and I think that I want to mass-load it.
The question is mass-load with what and, how much weight?

I’ve been looking at (for instance) a box of granite/marble slabs etc. from Lowe’s that are still in the box and strapped, or bags of steel shot from sandbags.com etc. on Diversitech pads.

Would those options “rattle” etc?

I know that the obvious answer is to experiment but I really would like suggestions first and there seems to be surprisingly little information on the subject anyplace.

Thanks!