Applies to UV processes only: Scanner glass will do the job IF it's indeed glass! All the flatbed scanners I've used (and seen) were fixed with acrylic lids and AFAIK acrylic isn't as transparent to UV as ordinary glass...

OTOH, even if your process doesn't need UV light, be warned about the fact that acrylic is way lighter than glass, therefore you'll loose some pressure which may translate as unsharp contact prints...

Regards,
Loris.