Have you ever heard of bending an Upper receiver?

[Klem]

^^^^ Yup ^^^^^

Never used spray rubber but sounds like the go.

I often use thin cardboard. It's abrasive enough to grip, but not mar the finish.

Have also seen rosin used in machine shops. I believe you can get it as grip enhancers for tennis and baseball (never used it myself).

Also seen an old-timer gunsmith use a thin piece of leather.