Irrational Number Line Games, LLC
Some chipboard, granny grating, and glue. That's all you need for some detention cells.
WEll, almost all you need. I cut some thin card strips to be frames between the cells and added some small squares to be latch/locks. Those details will go a long way to adding realism and they were pretty simple and quick to do.
As the main glue is drying, I test fit figures in the cells. I designed the cells for 1 1/4" squares, to give 1" or 25mm bases a little wiggle room. Also to accomodate more dynamic poses for figures.
I checked the cells and the pathways. The pathways were closer to 1", since I just took the width of the chipboard piece I had, subtracted 2.5" (two cells wide) and divided the remainder by 3 (i.e. two paths on the outside and one down the middle. That left me a little more than 1", some of which was consumed by the thickness of the granny grating. It's not very thick, but you're multiplying it by four.
I spray painted the whole thing black.
Then added various grays using splotch brush technique to make a concrete floor.
Not enough for a whole prison, but good for a reasonable detention center. And by using more of the chip board piece, a prison could be made modularly with the option to rotate the different cell blocks.