Baby gate for bunks

bed rails that clamp to the bunk/mattress.

We used a toddler bed rail for a few years on the top bunk. Something like This Munchkin. Find one the size that fits your opening and you'll be set.

How about a tight knit netting than can be installed with some good sized cup hooks. They might roll off the mattress but not crash onto the floor.

Our lower bunk is a futon mattress setup that can be easily removed. We set up a pack-n-play play pen in its place which worked well. You can also use it outside other times.

How about something like is adjustable width pet gate.

Here are some more carried by Target one of which has vertical wooden bars

I know, I know, it's a pet gate, But the same company that manufactures pet gates also manufactures the same gates for toddlers and charge more money for them.

I bought a similar pet gate to pen the family Chihuahua in the bathroom when we were not at home. I didn't use the wall attachments because it friction fit tight enough between the door jambs. I purchased mine as a pet gate and found the same exact one in a 2 pack at BJ's Wholesale Club sold as child gates in two different colors.

Google Munchkin Safety Toddler Bed Rail. Straps under the mattress, easy to put on/off.

Should be an interesting project. Are possible hinge points strong? Is there a window and, if so, does it have an emergency release?