I've racked my brain about this for weeks, and cannot come up with a solution that would validate a coin as a manufactured coin mechanism would.
I ended up buying a coin mech, mostly to see how it works and see if there is any kind of DIY solution to it, but it just doesn't look like there is one - it's too mechanical and there is too much bent, welded and twisted metal in there for a DIY'er to tackle. Its a bit like trying to build a DIY monitor, it just can't be done in a build yourself kind of way.
I've also seen the other "solutions" with paper, card and other materials, but shardian is right, they are all just a chute to guide the coin past a switch, and will not validate a coin.
I'd be really interested in anyone else's solutions to this, but I would also recommend bugging and begging anyone you can.
I went to Blackpool Pleasure Beach this weekend with my family, and there must be thousands of the damn things. I thought there must be a stockpile of these things somewhere that are old / broken or just a little TLC would fix.
Good luck buddy ...
EDIT: Here is my coin mech to show you what I mean ... The microswitch fits just below where the coin goes through, but I've removed it for storage because there is a thin wire to activate a credit when the coin passes over it.