Your primary objective is to keep the keys from being depressed. A single sheet of acrylic is ok because it can help spread the force across many keys at once helping to prevent keystrokes but it's not ideal because: 1) if it's too thin, you could still press only a few keys in a specific area if you type hard, 2) if it's too thick it's going to add weight onto the keys/keyswitches, 3) essentially putting all the key switch springs in parallel allows for them to support more weight without depressing but it doesn't take into effect how heavy your keyboard is, 4) acrylic can be sharp and if it slides around on the surface there's chance of scratching the finish or even slicing a finger if the edges aren't cleaned up properly.
Personally, if I was in your situation and looking to use a set up as you have shown and you're not looking to spend too much, grab some popsicle sticks and build yourself a little stand that goes around the bottom and elevates the bottom of the keyboard just enough to keep it off the keys (probably only 2-3 popsicle sticks stacked needed). If your only objective is to elevate the keyboard from the laptop keyboard for as cheap as possible, imo the popsicle sticks are the better choice because 1) cheap 2) don't run the risk of depressing the keys anyway 3) less likely to cut you or scratch the computer.
If you're in a location that would allow for something a little more unorthodox, I recommend a laptop stand to raise screen closer to eye level and put the keyboard underneath it. One of the biggest issues I have with working on my laptop is neck strain from always looking down at the screen. Might be worth considering if that's an issue you have as well. In some cases, Amazon has some cheap ones that fold up tight and can be thrown in a laptop backpack. Ultimately, they might both look a little silly to an onlooker but a good keeb and ergonomics should never go ignored.
