I have the Hogue Jr. on my P3AT. I simply went to the local bicycle shop and they pointed me to their dumpster...cheap inner tube.
One layer of tube did the job. I cut it a little shorter than the Hogue grip so that it does not show. But this helped even more:
I took a small section of rubber tubing (not the bicycle tube) and sliced it in half. Then placed it along the backstrap and held it in place with the inner tube. I lubed all of that with a little bit of water and soap then slid the Hogue over it. The finishing touch was taking my Dremel with a nylon brush and 'melting' the edges down to a rounded finish (I found this tip on this board). It looks great and fits my hand perfectly - I really like the raised palms...others don't. The rubber tubing really absorbs some of the bite of the recoil.
One caveat: This grip makes pocket carry a little harder while drawing because of the friction from the rubber grip. This is a trade off for the comfort of the grip. I typically carry mine in a Fobus paddle holster with untucked shirts but a Smart-Carry would work great too!