Here is a very simple yet effective neonate to small rosy setup. For a larger rosy, use a larger box. I use Cambro polycarbonate food pans from Smart & Final.

Soft pine shavings, some newspaper for a hide, and some heat tape. Very minimal yet provides all they need in terms of being able to thermoregulate while hidden.

I only have the water bowl in there for about 1 day per week.