If you can hang a bath towel over a pull-up bar or anything weight bearing then just use that, simple and effective.

Hey Anthony,
MY high-school coach used to have us do towel-pull-ups. They really work the grip but I think I got elbow tendonitis from over-using that exercise, so be aware that having to grip that strongly while executing a pull-up puts a lot of strain on the elbow. Might be better just to hang from the gripped towel.

I think that you could also (a variation on @dada good suggestion) make something with PVC pipe, a few angles, and wrap the whole thing in athletic tape to have a vertically oriented shaft type thing to hold onto.

The other thing I travel with often is this

The Block

it’s lightweight (I used to bring Rock rings until they accidentally got left hanging from a hotel balcony near Venice!) These are much lighter than rock rings and arguably more versatile.

Hope you figure something out.
cheers,
Steve

These look like a good option as well: https://furnace-industries.com/about-dry-ice-tools

I recently got my hands on a set of the Torr Trainers from Hex Climbing: https://www.hexclimbing.com/product-page/t%C3%B8rr-trainers

I’d highly recommend them, and I think it’s worth supporting a company that makes such climbing-specific training materials!