I think your biggest concern should be tight and strong joinery.

Disclaimer: I haven't hollowed chair seats before, but if it were me
getting started:

To use an inshave to hollow out, trace out the region to be hollowed,
and run one even pass through the whole region. Then trace another line
inside that region, like a contour line, and make another pass.

If you start with a region that's cut out of poster board, you can
duplicate it for all your chairs. If you get some practice so you know
about how far inside each region should be from the last, you could
make a series of poster board templates for each pass. The border of
the poster board could be the shape of a seat base, with the region cut
out of it. Just line up the edges and trace. Then for each chair and
for each pass, you know your region is properly positioned.