Here is the bottom line. The principle is called WEAK LINK SYNDROME. It has to do with the actual TRANSMISSION LOSS of the booth assemblies. Here is how it works.
IF, you design booth ASSEMBLIES with a certain TRANSMISSION LOSS target, say 25db, it makes no sense to install a window assembly with a TL of 40. The floor, wall, cieling and door assemblys would allow sound to transmit at a lower energy than the window.
Here is an example. If your booth was constructed the same as a STANDARD WALL assembly, it would make NO SENSE to install a window assembly with two planes of 1/2" glass. Or vise versa. It would make no sense to build a DOUBLE WALL.. 2 leaf booth, and then penetrate one wall assembly with a plane of 1/4" glass.
BTW, forget plexiglass...unless your booth is shiethed with cardboard. The key to transmission loss is MASS. Better yet...decoupled Mass.
Plexi can work, but you will want to make sure you dont get two sheets that are identical in thickness. (I don't know exactly why, but it is all over the boards here).
This doesn't take into consideration flanking paths from venting ducts, door seals, decoupling from the floor etc. They all have a bearing on the overall transmission loss. BTW, same concept goes for the door.
No one can tell you what will really meet your needs without knowing how your booth is constructed. Even the glass "decoupling" design will make a huge difference. And you can't plan that untill you know the weight of the proposed glass.
Maybe if you give as some info on how your booth is built we might be able to suggest a viable solution.
alright breaks over, back on your heads!