Not a great idea. You'd need a new clamping drain to put in a liner and preslope.Concrete is not waterproof, just not damaged by water. If you used concrete, there'd by no good way to keep water, body oils, soap scum, etc. to soak in. If you really want to stay cheap, get a circular shower rod, and a curtain, put a rainhead shower head coming straight down and a set of valves. Don't let the inspector see it, and replace it all when you decide to do it properly.