You need to get the husbandry right. The humidity is obviously staying too low and you need to get that fixed. A pic of your setup would help as well as a pic of the injury.
Instead of the towel on the screen you should make a cover that covers most of it. I used s cardboard cutout wrapped in foil. This will hold both heat and humidity in.
Next we need to evaluate your substrate. What is it? Cypress mulch is great for holding moisture and I would recommend using it if you are not.
Third is a great tip that I have been using for years now. Get some moss at the pet store. Dampen it and place a chunk in the hides. This will create higher humidity in the hides and done the snake is there most of the time it will greatly benefit the snake.
If you get those things done you should be on your way to having nice sheds. These are things I do in my glass tank and I have great sheds.
I'm a bit confused why you can not, and have not, been able to buy a tub. They are extremely cheap. What's the issue? Both a bag of cypress mulch or box of moss cost more than a tub so I hope you can afford it so we can get your snake's husbandry correct.