New BP owner here....

Based on what you already have going I would just recommend that you cover a good section of the screen to keep humidity in. I would use a piece of plexi-glass or a towel even. It would also help keep the ambient temps up so you could turn down the dimmers a bit.

If the shed needs help then your humidity is too low. I keep humidity around 60 on all my ball pythons (9) and not one has had a bad shed.

Edit: Considering its eating for you I would say your doing pretty well. I almost always have trouble getting my snakes to start eating.

To help keep my humidity up, I went to walmart fabric department and bought some of that plastic that comes on the big rolls to cover my screen top. I got the thickest stuff available and I think I got 1/4 yard which cost less than a buck and I have plenty left. I still keep a constant eye on the humidity but unless she is in shed I don't have to do much at all with it.