I have recently purchased a C41 pistol and also experienced the loose grips. I noted they are secured only by friction and at the slightest touch will slide back. There is also a bit of left to right movement which does not feel right when holding the gun. I elected to remove the one piece grip and noted three slide rails. That is all that is guiding/securing this grip.
For my solution, I wiped down with rubbing alcohol the back of the gun frame and the inside of the grip to remove any oil and grease. I took a narrow 3/16" strip of mastic RV window sealant putty and placed it on the back of the gun frame and spread it out with my thumb to a depth of about 1/16" of an inch. I reattached the grip and allowed it to interface with the putty. It showed no movement back and forth or side to side. With slight pressure I can slide the grip back to service the CO2 cartridge.
I have had absolutly no problems with this and it works very well. Tight grips brought on more accuracy.
Give it a go and let me know how it works for you.
I must try that on mine right now im useing electrical tape that works well but your idea sounds much better thanks for sharing.
Mike