The process is relatively easy. I start by filling in the hole with heavy gel medium and waiting oh-so-patiently for it to dry overnight.
I secured some keys and oddments to the gessoed CD using heavy gel medium (for the weight of the keys in this case), and after it was dry over-painted with white gesso twice.
I managed to twist John's arm, and he made me some wooden blocks to use for backings - they're 1/2" x 3 1/4" x 2 1/2", AND he primed them for me. What a treasure he is! Although the heavy gel is not yet dry, this back view photo gives you the general idea.
I encourage you to give it a try, using keys, dried vegetation, puzzle pieces, metallic embellishments, bottle caps, etc. No limits, all fun!