Building a mudbed for a shower isn't really all that hard, and even if you mock up the area and do a trial one, other than the work, the materials are really inexpensive...you could build 3-4 mockups and probably more for the cost of a manufactured shower receptor. There are at least two, often major benefits of building a custom one: most floors are not flat and this easily compensates for that; and, it's rare that the drain is exactly where you need it for a manufactured pan, and this again is easily compensated when doing a custom one. Building a quality, leak proof shower isn't really hard, but it is very detail oriented. There's many more than one way to do it correctly, but once you've chosen a method, it is imperative that you understand and execute the required steps properly for success. Mixing and matching methods can work, but are best left to someone with lots of experience, something a typical homeowner does not have, nor understand the ramifications of making mods to an approved method.
Should you decide to go that route, rather than having to build two mudbeds as would be required for a conventional, clamping ring shower pan, there are numerous waterproofing methods that would allow you to do this with only one mudbed, then a membrane, then tile verses mudbed, liner, mudbed, tile. I've put some info in the Tutorial section, but if you want to pursue this, I suggest you go to
www.johnbridge.com.