You could try checking some of the larger outdoor wooden hutches and just use it inside. But they tend to be very pricey and not nearly as large as they look in the photos.
Someone on this forum found a NIC cage already made for online purchase. Not sure if it was thru ebay or not.
I know there are tutorials for NIC cages that make them look like a lot of work. I've made many cube cages, but I never cut plywood or glued down any laminate flooring or used screws, etc.
NICs can be made much more easily than all that.
They aren't difficult to make and don't take that long either. I used rolled linoleum for a base and just laid it loosely directly on my floor. The NIC sat on the edges of the linoleum so bunny couldn't get to those edges.
I used scrap carpet on top of the levels.
A 1x1 fits right inside the grid openings and can work as an upper level support.
Zip ties make quick work of connecting grids.
