I suspect it's hard to find because, once you mount the towel warmer on the wall, it's only 12" off the floor which isn't much space to hang a towel. Mine is the size of a bath towel folded in half, so when I hang the towel, it gets the whole thing warm.

In my mother's old bathroom she had a wall hung electric heater and her regular towel bar was over it...it kept any towels she hung on the bar nice and toasty.

This company has some pretty small ones. I thought perhaps the swivel ones might work, if you could mount beside the window (the mounting piece is larger than your space) but the warmer bars then swivel into place under the window. But they also have custom made ones.

Here is a link that might be useful: custom towel warmers

I would like to see some Amba towel warmers in person but cannot find any information on dealer showrooms.