I had a lapidary cut me an oval rock crystal to size, with a dome,
yup basically a cab.
It occurs to me that lapidary firms already supply exactly this sort
of cabochon for use with opals, to make opal triplets. If she can
find the supplier of these clear caps, she’s got her flat backed lens
shaped cabs ready made, rather than custom cut…
Also, back to the OP’s original effort to try and find ready made
optical lenses, there ARE lenses cut with flat backs. called
plano-convex, they’re flat on one side, convex on the other.
And another idea would be eyeglass lenses. Those, are often convex
on the front, and concave on the back, which would give even more
clearance under the lens…