From the photo MarcH linked to, it might be tough to squeeze in extra stones. But there’s a fair bit of empty space, if you were to re-organize it.
As an alternative you could buy a block of Kaizen foam and reorganize the pieces and cut out to your liking.
Example of Kaizen foam: https://www.amazon.com/Fastcap-Kaizen-Foam-57mm-Black/dp/B007J2JJYK/ref=pd_sbs_201_1?_encoding=UTF8&psc=1&refRID=Q6AHHXWCF1CCH3KR7421
(but do a search to find a block of the right dimensions)
The hard part about cutting a solid block of foam is if you need flat-bottom holes or slots that don’t go all the way through. The foam is generally too soft to use a dremel tool to get a flat-bottom cutout. Since the Kaizen foam is layered, you just cut the outline to the depth you need–instant flat-bottom cutout.
PS, if anyone has found a tool or bit to carve out clean flat-bottomed slots in regular soft foam (not Kaizen foam) I’d like to hear about it.
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