[mangoandmimi]

if you don’t want something too big a ball pit (Amazon) works well for robos and is probably the cheapest option, aside from a cardboard DIY one. They generally shouldn’t chew it but do need supervising. I like ball pits as they have a base already and fold away in one piece. Don’t think they’re as suitable for other species but it’s good for robos. I recently also bought the skyline ranch one off zooplus which is good too but more expensive.
Not sure where you’re from but can you get hold of something similar to a Ferplast Aladino carrier in the medium guinea pig size? That’s roomy enough and very secure. You’ll definitely need a carrier for if you ever need to take Monty to the vets too.