That's actually a cabinet front with a pull out drawer insert. The open cabinet front can be seen to the left of the drawer. Can you use the same cabinet front that is in the bathroom even if it's not close?

Another option would be to place shelving in that space for rolled up towels…. Just a thought.

Yes I see the open cabinet front! :) Shower is across from bathroom vanities/cabinetry- shower bench will be about 36" wide (so drawer could potentially be 36" deep). Shower is being built and thought it would be interesting to maximize space under the bench with a custom pullout drawer, however, thought it would look silly with just a cabinet panel. Was hoping to possibly see some interesting ways people integrated this idea. Thx.

People in the Kitchen Forum make drawers like these to put underne’er built-in bench seating in nooks. The long, skinny drawers have the nickname ”morgue drawers” there. i do not know if this is a common term or unique to the kitchen forum.

I'm wondering how the heck one gets into that tub?!

I was looking to do the same thing. Make use of the space and make it a pull out style drawer not a cabinet open and then pull out a drawer. More like one of the revashelf ideas.