Table Casting Advice

I have made this sculptural coffee table from 3D prints and fibreglass that I am filling with a thin layer of polyester resin and Q Cell to create the top (from multiple thin pours), which I will then back with fibreglass for strength. I plan to use this thick mirror glass as the base, however with the table upside down you can see there are small gaps between this and the glass where it is not completely flat (mirror makes this appear significantly larger than it is. Two questions:

1: what mould release should I use if any on glass?

2: what do I use to block the holes? Would a clay barrier around the entire structure work? Or is there a way for me to also stick these parts to the mirror and block the hole whilst I cast the top?