Completely agree! Depending on your miter saw fence setup you might be able to get enough support (with zero clearance or near zero clearance fence) but you can’t go wrong supporting the cut this way. One other option would be to lay the liner flat on the saw and tilt the saw so it is supported underneath by the saw’s table. You can also lay a little piece of scrap plywood or similar on top and then apply pressure to that with your hand while still staying away from the blade.

Absolutely great idea. I just did the cut with a handsaw ended up OK but , not as good as it could’ve been using your technique. Another wonderful idea. Thank you!!