This type of thing requires much research. I haven't done mine yet,
but I've read about a dozen how-to's on the internet from "the mudman"
and such. They're all very informative, and they describe the process
well...one even showed how to create a "lipped" version for handicapped
folks, sounds like what you want to do as well.

Building your own floor doesn't *look* all that simple. You've gotta
do all kinds of stuff with mud, such as slope it, and you HAVE to use a
membrane and a special drain. Also, you can't put any holes in the
membrane, so you've gotta use construction adhesive to hold it to the
mud, and you have to reinforce the mud with mesh, but don't leave any
mesh poking out or you'll puncture the liner.

There's a huge advantage to this extra effort though, you can make it
look however you'd like and it's way cheaper than buying one of those
$500 tile redi bases...and you can make it any size you'd like.

common sense tells me that this, like anything else around the home, is
not impossible as long as you research it first and actually take a
while to think about what you're doing and be patient while you're
doing it. It always helps to bounce it off the fellas here, or
buddies who may be professionals before you go at it.

Remember, the purpose of the shower stall is to keep water where it's
supposed to be. Just as long as you don't lose sight of that and think
about all the different ways water COULD find it's way to where it
shouldn't be, you won't make a critical mistake....cosmetic mistakes
are just part of life.