Houses - I have used this one before. It’s about the same size as a labyrinth house but just open inside. Because it is tall with such a large door, it works well when sat directly on the cage base with the substrate piled up around it abd some inside. This effectively reduces the height, makes the door smaller (but still wide enough) and also means it’s supported so can’t sink. So it is part subterranean basically. The roof lifts off so you don’t need to take the house out abd can just check inside or remove the litter tray. A corner litter tray inside works well and they will use it. Also keeps the cage clean and dry so less bedding used as you can go much longer between cleans (3 months or more with spot cleaning and a litter tray).
The roof also doubles up as a platform or somewhere to put a food bowl. I had this before I got the Rodipet labyrinth house. It’s smooth wood and that shouldn’t cause any allergy issues. But if you wanted to seal it or make it wipe clean inside then a pot of plastikote would do that (pet safe and virtually odourless).
It comes in clear or colours. Can’t find the clear but this blue is nice
https://www.amazon.co.uk/Plastikote-...l+paint&sr=8-4
Ferplast Sin 4647 Rabbit House, 37 x 27.7 x 20 cm: Amazon.co.uk: Pet Supplies