I have a habitat (which I call the mousehouse) that is two IKEA Billy bookcases double stacked. And then split so the lower 2/3 is one enclosure and the top 1/3 is a separate one.
The top 1/3 I have previously used hamster-wise, so no questions there. But in case the info is useful for comparison, the whole front is an opening mesh door, and there's a small (2x1cm) ventilation hole at the top of the back too.
The front of the lower 2/3 is just over half glass, just under half mesh door. Currently this is the only ventilation. For a hamster I feel I should add some ventilation at the back, but I'd like some views on the size and placement of that.
Here's a photo of the lower section set up for the previous inhabitants:
[NB I'm intending on using floor space only and retaining the shelf as an inaccessible store.]
My thought at the present point in time is to cut a window the full width of the mousehouse at the back underneath the shelf. That way I can get an angled through draught. But without it actually being a draughty place and also without limiting substrate depth in the future. However I'm not really sure, so input would be useful.
Because it's bookcase construction, ventilation at the sides is out.