Making flat CF sheet on a large sheet of hardened glass (like your shower door) is a very good way of doing it.. works brilliantly..
You may want to support it with a wooden (plywood) frame to make sure that you do not break the glass.
If you have the skills, then you can build a heated setup, by using a hot air blower and channel it under the glass, that will speed up the hardening and also make post-curing at high temperature possible.
Make sure that you use a good release agent on the glass, Epoxy sticks VERY WELL to a dry clean glass surface, it must be treated with something like EC "Easy-lease"
You can polish the easy-lease on the glass to a mirror-finish, making the glass-side of your sheet a mirror finish as well ...

I make double-sided "mirror finish" sheet, by simply taking off the peel-ply on the top side of a sheet made on the glass, coating it with laminating resin and turning it over on the glass sheet. but remember to de-gas the resin first, and remember to clean the glass, and apply and polish a new coat of easy-lease.
You can Vac-bag the sheet again, but I normally just use a second sheet of glass and put some weight on top.
Good luck!
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