Stop guinea pigs from gnawing?

So I have one castrated buck and two females. They are trusting and can do lots of tricks. They are absolute gems. The oldest and most trusting one has recently started chewing on the plexiglass, which is not only annoying, I'm also worried that something might happen. First of all, the basic enclosure with the plexiglass is 1m² (which is too small), which is why they have to cross a bridge (which everyone can cross) into a larger enclosure (4m²). There they have lots of houses, tunnels and also fresh branches, a nibble house, a few boiled pine cones, etc. The others also use these things to wear down their teeth, only Trude (the oldest) wants the plexiglass. They also get fresh food three times a day, and have hay and herbs available all day 🙁 Any tips??