I found that kydex holster material works well to increase a grip size. Similar to fuedist's strip of leather, get a piece of .100" or .125" thick kydex and cut it to the same width (or slightly more) as the backstrap. Heat it w/ a hair drier until it softens and mold it over the backstrap wearing gloves. Now finish the edges so it is just slightly under width. Use no-slip stair tape slipped under the grip panels to hold it in place. This is what I did to my Beretta 92X Defensive to make the straight backstrap similar to the curved version.
20220923_083817 by craig stuard, on Flickr
To increase the entire grip length just make the kydex piece long enough to go from top to bottom. You can also just use the tape. Cut so you have 2-3-4 layers on the frame and then make the top layer wider so it can slip under the grip panels. A little hair drier heat helps shape the tape and seems to kind of set the sticky a little more permanently.