I've been in a similar situation. When I got my Monster Bash, it had a new-ish Cliffy that looked good, but because it was installed over a little damage without any repair, it eventually bent and split. I ordered a new one, and tried to install it right, but I also didn't want to do anything that wasn't reversible, because the playfield is otherwise in pretty good shape.
So, my approach was to use QuikWood (ordered from Pinball Life) to fill in the gap, but to install it on the Cliffy rather than on the playfield. (My pictures should explain.) The way I did this was to use saran wrap to keep the playfield totally clean and unmolested, and to add Quikwood to the corner of the Cliffy, then squish it into place in the playfield, add/remove material, re-sqish, and repeat until I thought I had just the right amount of material filling the void.
This was totally my own technique, but it was definitely a success. The new Cliffy has held up really well, and I didn't do anything unreversible.