THis doesn't sound like rocket science. I would just put in a mud-job, tiled shower base using standard techniques. This would involve making your curb, probably out of masonry, and having a copper pan made by your plumber to fit the space, then the special drain is put on, one that directly drains water and also has weep holes below for any water that makes it through the tile etc.. Then just fill with mortar and slope the whole thing toward the existing drain location, tile everything, put on your door, and you are done. I have done many like this and most plumbers that I have dealt with do these all the time. Am I missing something? With a custom pan and mud job base you can just slope everything down to the existing drain location.