Most keyboards utilize a series of grippy feet designed to keep it firmly planted during use. A number of factors including size and placement of the feet as well as the material they're made of and the surface of your desk or table are all instrumental in determining how well a board stays put.
Weight is also a significant contributor to the equation. More often than not, the heavier the board, the more likely it is to remain stationary... a fact that's no doubt true for Reddit user ipee9932cd.
In the wake of not being able to find a suitable keyboard case, the Redditor did the next best thing - created one out of concrete.
He noted in the comments that he knows nothing about concrete. As such, he chose not to use any rebar for stability sake and thus, doesn't handle it using just one edge. The chassis weighs around 12 pounds and was sealed with food-safe countertop sealer. A thin, smooth coating was left on top so he can polish and wax it.
While certainly not for everyone, the project is an excellent example of the sort of creativity that can be realized when thinking outside the box and could serve as a unique conversation starter for someone that works in the concrete / home building / remodeling industry. Just imagine what could be done with a scrap piece of granite or some other high-end building material.