Good, bad, or mediocre idea

Hello, longtime wood butcher-first time tiler here. Im currently renovating my small bathroom on a somewhat tight budget and have an idea for a DIY shower pan thats probably a terrible idea but could potentially work.

The idea:

I have a half sheet of 3/4" marine grade plywood as well as a half sheet of 3/4" PT plywood leftover from a previous job. Could i potentially use one of those plywood materials on top of the concrete subfloor, embedded in a layer of mortar and installed at the appropriate slope, topped with cement board, a waterproofing membrane, and a linear drain at the end to make my own redneck showerpan without running into issues with water pooling/leakage down the line?

The rationale:

Because of the odd size of the shower itself and the constraints of the rooms footprint using a prefabbed fiberglass/plastic showerpan is not really feasible without having a custom one made, using a schluter showerpan that would work with the current drain location would result in way more waste than id like relative to the price of the pans, and i lack the skills/money required to make a mortar pan or pay someone else to make one for me.

The area where the shower and the necessary plumbing are located has a concrete subfloor with the drain located closer to the wall, so relocating the drain would be a royal pain in the ass to accommodate a prefabbed pan that would work in this situation. The dimensions of the showers footprint are 45"*30", and the showers footprint cannot be increased, it could be decreased but its already pretty tight as it is and any space sacrificed would not be usable for anything else.

Any input is appreciated! Please tell me if im an idiot or not.