Why don't you shim behind the wall to make it square, then find the right size door that will fit. Don't make the door fit the wall, make the wall fit the door. I don't think a filler piece would look that attractive. In a pinch, if this is the way you want to go, you could make it yourself- simle DIY project. I'm sure HomeDepot has all the materials you would ever need.

My almost finished main/guest bathroom remodel used to have a standard bath tub with a sliding frosted glass shower door. I hated that thing. Whenever I wanted to take a bath, I felt claustrophobic (even though I'm not). And getting in and out was oddly uncomfortable, even though the opening was plenty large. The only thing I liked about the shower door was being able to lock the dog in when I needed to give her a bath. I could also see the doors as being good splash-prevention if you had children showering. One other thing to consider is your selection of a swing-open door vs. slider. Using a swing-open type door will mean that the door will be closed while taking a bath. For me, that would be uncomfortable. I like to be able to rest my arms on the sides of the tub (to relax) and the door would be in the way. Personally, I would not want to have a glass door on a bath tub ever again. I would suggest you do a test-drive of some sort. Perhaps try bathing with the shower curtain closed or even putting up "doors" of cardboard or some other material to simulate the feeling of having actual doors. Maybe it will work better for you than it did for me.

I would do regular clear instead of the low iron glass (less greenish). The price isn't worth it and the only time you can tell the difference is when your looking at the edge of the glass. As far as the coating, it's amazing against hard water. You won't have to squeegee every day. Hope that helps, I work at a glass company.

Typically done that way. You need to drill to attach, the key is few holes as absolutely possibly and to load it with silicone before sending the screw in.