No special glass required as far as I know. The thickness of the glass is another matter, you want it heavy enough that is squashes any curly 35mm negs flat.
I have my own theory on these... I have a 'proper' 35mm contact print thingy, which gets 6 strips of 6 negs on a 8x10 sheet of paper, however the method it uses to hold the negs is a pain to use. I can do about 5 proofs using a plain sheet of glass by the time it takes me to load this other gismo. After many, many years of using it , and cursing it every time, and a move to MF I went to a plain sheet of glass. I've tried printing thru the neg sleeves and I'm convinced you don't get a sharp image so I take the negs out of the sleeves and lay them on the paper then place my glass over the top. If you've manage 37 or 38 frames on a 36exp, then with a bit of wiggling you can get the majority of the image of the extras on a 8x10 sheet. Here's were my 'trick' comes in. The glass I use is actually wider then the enlargers baseboard, which makes it easy to pick up without sliding across your negs, and keeps finger prints way away from the area used to the negs. One piece will cover all your 35mm, 120 and 4x5 needs.