I found a product in the sporting goods section of Wal-Mart called "Soft Shock" by Wilson. The price is under $3.00. It is designed to be wrapped around a tennis raquet handle. It has a thin foam core attached to a glued rubber tape.
I built up the back of the P-3AT grip with two pieces of Soft Shock (you could use more if you like), and then wrapped up the entire grip with a good non-slip grade of electical tape, avoiding the magazine release button. Here is a side view.
Here is a view from the rear. The foam core is white, so I had to use a little permanent black ink on the ends of the foam core.
This was a cheap, easy modification, and the pistol feels more comfortable, and easier to hold on to. The process is also completely reversable, if you decide you don't like it. For more ideas about improving your grip on the P-3AT, visit the Kel-Tec Owners Group or the The Kel-Tec Range
Update: I couldn't leave well enough alone, and decided to add a Hogue Handall Jr. that I found on EBay. I stretched it over the electrical tape (a really tight fit!), trimmed it a bit to keep the magazine free, and then taped over the Handall Jr. to make sure it wouldn't slip. Here's a scan:
More recent update (Jan. 11, 2005): I took my P3AT out shooting yesterday, and after 50 rounds had rubbed a nice sore at the base of my thumb! Here's a picture:
So I added some more Softshok to cover up the frame corners, and then wrapped it on tight with electrical tape. Looks like this now...
I'll let my hand heal up for a couple of weeks, and try it again, and see if the extra cushioning was helpful. See y'all later!