You can also try this thing: http://www.vinylflat.com/vinylflatproducts.html
. It is made to be used with a normal oven or you can spend an extra $60 USD (I did) and get the Groovy Pouch. That pouch is a haavy bag lined with heater elements that warm the LP in the Vinyl Flat up to a controlled temperature.
I have tried this on an original Apple first pressing of Abbey Road and it got it dead flat without harmimg the vinyl. Since that LP I've tried it three more times, all with success.
It is a lot cheaper than that thing that costs around $2000 USD and it is cheap enough to make sense to own. I have tried the two pieces of glass approach also and it works OK but you have little control over the temperature that the LP will get to.
The Vinyl Flat holds the LP dead flat and the groovy pouch allows you to heat it at a controlled temp. It takes several hours at the temperature the pouch heats the LP to but I have yet to loose a LP to the sun or oven doing it this way.
I have mine hooked up to a digital timer so I can set the duration of the heating.