Depends what tape and how many layers of it, but I'd say 2-3 layers of bog standard LX tape wrapped right around the grip over the bit of the pad which isn't the pressure switch (or the cable just next to it if the pad's too small) should see you right. I wouldn't bother using camo tape to hold it if I were you - it's not that sticky, but I spose you could wrap some around it over the top afterwards if you like the look of the stuff on your gun.
You could also use 20-25mm flat elastic stitched into a loop - if you make it yourself you can make sure it goes over the grip tightly enough to hold the pressure pad. Or there's Velcro...
I have some OD 25mm elastic which I could cut a piece off for you if you like - say £1 inc P&P - just measure the circumference of your grip and PM me. I'll send you a bit that long so you'll have enough to stitch. Or I spose I could stitch it for you, if you send me a pic of the grip and the pad so I know what it'll need to be like - £2 ;0