Yes a variety of clear substances like white glue could be used to make windows or vision blocks... However, the ones you're talking about on the Panther are retractable periscopes therefore they don't take up the entire space under the armored cover. it's more of a block or triangular shape that sticks up in there that would be glass on the outward face and metal on the rear and side surfaces... Check this link out.
http://www.armortek.co.uk/forum/viewtopic.php?p=2627&sid=a790e4908cc68bea29c83ba50315cad5