Jim Conlin wrote:
Evan, i've thought that the 'puck' that's in place of the core under a
loaded fastening only needs to have the compression strength to bear the
fastening loads and be non-decaying. In that case, most any high-strength
bog will do the job. Why glass?

A layer of very thick bog will not be very strong unless you reinforce
with milled fibers. Even then it won't be well attached to the skins
to transfer loads to them. This is because there are often sideways
shearing forces that need to be taken into account too.

Evan Gatehouse