Never heard of the aluminum thing...I don't recommend it though. I think varnish is a bad idea as you are going to be heating it up a great deal and it might give off chemical vapors...it'll hit your fabric before it hits your lungs.
As far as what to put between the cover and the wood, I used my hubby's industrial staple gun (all of once before I made him take over) and stapled on old bath towels...one to each side and then slid the cover on like a pillowcase and stapled that in place as well.
Good luck!
Melissa