Hi there,
I've got a large hutch with no bedroom section (after lots of trial and error with bunnies making it clear via trashing of the hutch that they didn't appreciate my bedroom creating efforts
So I've insulated under the floor (it's on 3 ft tall legs) with foam boards cut to size and then all around the outside top bottom, back and sides has a thermal cover made of silver foil bubble wrap for good measure. The hutch sits within a solid roofed and partially tongue and groove cladded aviary.
Inside floor is lino and on top is newspaper and they have a large litter tray at each end which are filled with newspaper and hay on top. Then a hay rack as well above each (which they rarely touch
I did have a platform in there but took it out because they never used it for anything.
In the winter I tried putting in cosy cardboard boxes filled with straw and snuggle safes and they would go crazy chewing it all up and throwing it about the place into water bowls etc. and would push it and block their pipe (and they used to use bedroom area as toilet only in the old hutch as well and sleep in the open section which is why I deliberately didn't put one in new hutch)
I ended up using one of those large grow bag trays and it fit pretty well between their 2 litter trays covering all along the middle 'floor section' of the hutch. I lined with newspaper and then that whole space filled with deep straw and 2 snuggle safes buried in. Success, they really love this setup, although will still wee in it all a little, but the tray aspect makes cleanup easy as like the hay trays just roll it up and bin it. It's never got below 0 in there over winter and so far hasn't got overly warm in the recent hot weather we've had. So far so good.
Hutch has got 1 solid door and 2 meshed doors and they have nightboards which fit into the mesh door frames, with 4 fairly large ventilated sections which are left uncovered. The vents are really important otherwise the hutch will get condensation and mould.
I'd also try and give them access to their run 24/7 if you can :thumb: and this will be important in summer when it gets really warm too. I've been awake at 3am a few times and seen them all sat out in the freezing cold, lol.
Good luck and apologies for the essay