Hey,
When I was trying to get my roll-in done I discovered a few things. Home
Depot and Lowes have catalogs with various types of pre-made i believe
fiberless roll in showers that can be ordered through them.
My x gf had one put one in and it was pretty easy to install. All you do is
build a stud(2x4) frame the size of the insert and slide the insert in.
One prob was there was still a small lip to get over to get in shower. This
could be solved by dropping the floor height where the insert is to be
installed by 3/4 inch then it would be an even entrance pretty much. Just make
sure you get a big enough shower.
ron c7