I've used blu tack, it is dirt cheap and easy to use and measure. The brand I've used recently even comes pre-grooved to pieces, so really easy to measure the weight. I've lead taped under the grip too, and used sorbothane grips (and the other heavier than regular non-leather comfort "gel" grips from Wilson, Head etc).
Silicone is a devil's invention, having had to use to seal bathroom corners and such, I will happily stay away if I can. (I even had see-through silicon (off ****) nose pads on my eye glasses for a while... they burned a permanent red mark on my nose... so no thank you!)
My Dunlops F3.0 and M3.0 only have a thick sticker at the butt, I thought if you took it off, you'd get access, but no, it is sealed off. Easy to cut an opening with a knife, though, and it seems the sticker still holds in place easily after stuffing the blu tack in.
I have the tendency of messing around with weights, so want to keep it cheap and simple.
My main rackets basically have around 60g of added weight at the grip end. Using sorbothane, lead tape, overgrips, blu tac...