it really isn't a problem to have small bumps and wrinkles. i just start at a top corner and work my way over and down, using thumbtacks or tape.. whatever is lying around really
you only want to avoid tin foil... it can cause hot spots because it can reflect in funny ways and can magnify light and burn your sweeties... as long as it doesn't resemble that and there is a nice EVEN distribution of light... you'll be all good.