I have tried a few things...
A Pachmayr grip and 1/4" thick dense foam strips layered under the grip:
Four layers of bike tube and split plastic tubing layered on the backstrap under the bike tube:
A Hogue grip and three layers of split plastic tubing and two layers of bike tube under the grip:
I tried foam strips first but the slip-on grip seemed to compress the foam over time and as a result the grip lost the added depth. I ended up with a combination of plastic tubing, bike tube, and slip-on grip.
I used three layers of 5/8" O.D. x 7/16" I.D. flexible plastic tubing, cut into 1/2" wide strips a bit longer than the backstrap, glued the strips together with Goop adhesive and clamped it until dry. Then I aligned the glued tubing along the backstrap and put on two layers of bike tube then the slip-on grip on over that. Lastly, I trimmed the excess plastic tubing off at the bottom of the grip.
Using strips of material under the slip-on and/or bike tube is kind of neat because you can experiment with different numbers of layers of material and at the same time you don't permanently alter the gun.
My first attempt shows three strips of 1/4" thick foam with a Pachmayr grip over the foam. Notice how the foam has compressed (more at the top where grip and recoil cause more pressure, but quite a bit at the bottom also).
The second attempt shows the gun with three layers of tubing and four layers of bike tube.
The last photo shows my gun as it is now with three layers of plastic tubing, two layers of bike tube, and a Hogue slip-on (I like the finger grooves).
You will notice that the overall length of the gun isn't any deeper than it was before the modification due to the depth of the beavertail on the grip.
I got a couple of bike tubes free - went to a local bike shop and asked for any bad ones they were throwing away.
The 20 x 1.75 - 2.125 worked best for me.
Bobo