The ‘traditional’ answer was sawdust. (or old newspaper. I’ve seen
If it were me, I’d look at soldering up a ring of silver wire to sit
under the worst of the bottom crowning, to stabilize it.
I’d also ponder using sliced up sections of plastic milk jug in
layers. That’s waterproof and reasonably stable over time.
The real question is why you’re using this stone at all? It’s going
to be a nightmare to set, and unless the top face is a whole lot
more interesting than the sides, I can’t imagine it’d be worth the
Unless there’s sentimental value to the customer, if it were me, I’d
go look for a better cut stone. Or grind the bottom of this one
flat, if I had to. Wet-&-Dry sandpaper would do it. (wet)