I have printed 100s of prints out of PA-12 and PA-6 CF on a S5. I have found that just Elmer's glue stick (I use this) or like gr5 said the woodglue mix works great, even for very large prints.
Not that you want to do this but you could but you could... Go to your local hardware store and see if they can cut you a piece of glass for the size of the S7. The other option is to get the S5 build plate and binder clip it to the side. I prefer printing on glass for the most part with just about everything. If you switch to glass you will be able to use the magigoo and it will work perfect. I would try glue on the flex plate.