The commercial flateners are a rip.I have yet to try the put it between glass (with paper sleeve) and think that more weight with bricks or granite is good idea).But know this is you have a nasty heat warp it's not going to work,I had a Lou Donaldson "Blues Walk" that I too cover to concert for Lou to sign.Got home and open shade had produced a nasty 1" heat warp.The tighter (smaller area) of warp and it's depth can be to degree that nothing will work (as that $2K company tells you).Gradual warps that don't 'kick tone arm might work.But set at lowest heat of 200 degrees for wide mild warps and cross fingers.
Chazz
Chazz