Just be aware that a 2 3/4" shell's overall length can vary a bit before they are fired depending on the crimp style. They are 2 3/4" after firing them. Before you fire them they are roughly 2 1/2", +/- some. And if you're also wanting to fire 3" shells getting the plug the right length needs to be pretty precise. If you can just barely fit two 3" shells in the magazine you're probably OK. But if you have much room left over with two 3" shells you might be able to squeeze three 2 3/4" shells in the magazine with some brands of shells. And that could get you fined even if you don't have 3 loaded.
I've cut a few, including one made from a tree branch I whittled down to fit while in a dove field. I had forgotten to put the plug in my gun and had to make one before getting in trouble. I'd advise cutting a little longer than necessary and use the trial and error method to shorten it some as you check for fit.