I had a lot of problems with mine falling off, too. I notched out the rubber on the back of the pedal, I tried double-sided tape…..I tried RTV. I’d eventually hit it with my shoe or something getting into the car, and it’d go rolling onto the carpet. Imagine if you were ever in a situation where you NEEDED to use the e-brake, and you slip right off of it because the pedal cover falls off. No thanks.
I just recently had the idea to drill a small hole on the center of the pedal cover, and drill a hole in the pedal or arm itself, and secure the pedal cover with a button-head Allen screw. I used a 4-40 stainless screw, you hardly even notice it is there. I used a 4-40 nut with Nylon insert on the back, so it wont vibrate or work its way loose.
Pic of when I had it on my bench at work: