I'm thinking this is the design I'll end up going with.
Here the towers are arranged in their normal arrangement.
The trolls sit facing their puzzles with the bowl to one
side. In this arrangement, they can easily access both
the puzzle and the bowls. Also, this arrangement allows
for a reasonable compromise between very large towers as
is the case where the puzzles are outside of the trolls
and very small towers as is the case when the trolls sit
outside the puzzles. The largest disks I imagine to be
about 4 inches across. The numbers one top right are two
possible sets of disk sizes arranged by 1/4 inch and 1/3 inch.