NT wrote:

acutally mite make more sense to use epoxy to make up the bulk, using
some sort of form to hold it as it sets.

If I were doing that I'd mix in lots of microfibres to make a stiff
enough paste that it didn't slump. I've used this to fill holes and
cutouts in both horizontal and vertical surface. Support is needed under
the horizontal ones, but the paste stands up on its own to fill holes in
vertical panels.

Pete