Great Topic! Certainly torch safety is a big problem when it comes to sweating copper pipes, especially inside the walls. There is not much worse than having the fire department show up at your job site....
I don't use the sheet metal between the pipe and the wood technique for the simple reason that sheet metal conducts the heat. If that sheet metal is red hot on one side you can rest assured that the side against the wood is red hot as well.
The flame protective mat is a much better solution as the conduction of heat to the other side is very slow and a lot less likely to reach a dangerous temperature even if you sweat pipe very slowly.
There are a number of other things you should have on hand as well for a good read check out the following link. Plumbing Torch Fire Safety
If you still aren't comfortable with the proximity of combustibles to the joint and you can't sweat it and put it into place, then sweat in a better location after, you can always use a SharkBite Push-In Fitting instead.