For what it's worth, as this as already probably already been said by Toad...
1. Yes, the whole thing can be set up before hand. I'm sure Toad has revealed the 'quick setup' item.
2. My method is to use a silver coin - i.e. 10p- for the 'melt'. They look much more convincing colour-wise.
3. Ask to borrow a 10p and a 2p
4. Have the loaded gimmick rolled up CAREFULLY, banded with the rubber band. They are in your pocket.
5. Palm off the 10p, and conceal the 2p. These are in separate hands, although the illusion to the spectator is that both coins are in the hand that the 2p is in.
6. Go to your pocket to fetch the rubber with the hand that the 10p is in. Ditch the 10p as you take the rubber.
7. Whilst under guise of using both hands to remove the rubber band, bring the 2p over to the rubber. Immediately now hand the rubber band to the speccy, and ask them to bring the glass closer. 2 instruction to the speccy mean their mind is elsewhere for a second...
8. Place the rubber mat FLAT on the table, with it's 10p and the borrowed 2p on top. The 2p can be stacked offset on top of the 10p. Careful - if you're not gentle, the rubber will tear.
9. Pick up the whole sheet, with the 2 coins, as one item.
10. Place over the glass, and ask the speccy to stretch the rubber band over the top, sealing the glass.
11. Ask them to name one of the coins... use a magician's force to force the 10p... i.e. if they say '2p', give them the 2p, if they say '10p', GIVE THEM THE 2p

and say "OK, you chose the 10p, that's the one we do the trick with' etc....
12. Perform!
Hope that helps.