Improvising is "at first" an idea that crosses every coach's mind at the youth level. I've seen gym mats folded ....to kids practicing off ramps usd for handicap access, and so forth. Basically, trying to simulate the frontal slope of a mound is the desired result here.
However, if your trying to start your season with 12U, a flat surface does offer a better chance to work on the basics - form, delivery, accuracy and so on.
And while we're not talking pro ball here, portable mounds are built with definite design considerations. They have a non slip surface that's made and applied so as not to come appart with repeated landing. The bottom surface has a non skid texture so the entire unit doesn't slide accross a gym floor - which is a very deliberate safety minded point in manufacturing, and the bottom is also designed so it doesn't scare or mark the gym floor.
Another consideration with making something "make-shift" is the legal aspects of this thing and its users. If you bring in something that you made at home, or, you put together some stuff from your institution, school, league --- you're not a designer of sports eqt, your not a pro contractor of sports eqt, and so on. You should chedk with your insurance agent before starting this.
I offer this advice because I've witnessed some really strange accidents in my time -- and when a son is hurt, parents seem to have a very narrow window on the world, not to mention some guy standing next them wispering in their ear saying ... "!!"