I used outdoor pens in the summer when temperatures were above 50 at night and indoor pens the rest of the year.
Floorspace was 8x8, obviously the pens were walkins so height was 6' outdoors, ceiling indoors. 10x10 would have been better for these big lizards.
Your other option would be free roam of a room. We keep our family's pair of green iguana's this way- free ranging in the house. We have a south-facing bay window that gets lots of light and heat from the sun and set them up with a couple of basking spots in and near the window. I don't know if the rhinos could be housetrained though- the greens are really easy- we just haul them off to a plastic tub with a couple of inches of warm water once a day and they poop and we then wash the iguana in the shower and then the tub, pouring the iguana mess down the toilet, rinsing, repeating and then washing the container. Once used to the routine they go within a couple of minutes of being put in the plastic tub and the whole routine for one lizard takes about 10 minutes including cleanup and washing up.
But rhinos- I don't know. Green iguanas are clean- they usually leave where they bask to poop, given the option. My rhino iguanas were more like bearded dragons- often they just crapped where they happened to be sitting when they got the urge. LOL So I don't know if they could be house trained or not. Probably though- they will poop when introduced to water and that is the key.
So- if the size of the adult enclosure doesn't scare you off- it will take a couple of years before they can really use a big enclosure like that. They are best started in something much smaller- Mine stayed in a 30x30x4' long and then 30x30x8' long before being moved into something larger. Adult size enclosures for iguanas usually are best about the time the lizards reach 30 inches or so. Not coincidentally, that's when people usually dump their green iguanas into rescues. So think carefully before you leap into it, and commit to the long term- these lizards deserve that.