Could you build a small return of 3 1/2 inches? Don't know how elegant it would look, and bear in mind costs of waterproofing and tiling etc? Cost/benefit vs custom fit shower door?

what would i use for a small return?

Standards and materials do vary from country to country, but here we would install a product called "alba" - a kind of stiff polystyrene block that you can saw and put in place. It is totally waterproof (except for the joins). Maybe US houzzers can step in.....?

A 59" or 60" wide sliding door is common, Kohler/Sterling is sold at the Home Improvements stores for around $350.00. Buying one of those would be your best bet in my opinion