Dr of BBQ
It looks good.
The foam will fill your hole but if you do ENOUGH searches on line you’ll find a proper plug. The choice is yours the foam is good stuff, but a pain in the ass to clean up and I find it hard to control. It always seems to go everywhere but where I really want it.
And then of course you use a little bit and the can is wasted because the line is clogged. But it’s fast, easy, and relative inexpensive.
When I just reread your post as I understand it are you going to drill a smaller hole in a 3/4 inch piece of plywood and lay it on your floor and foam from the bottom? If so that will work. You don’t even have to take the fittings apart just drill the hole with a hole saw and and then use any saw to cut the hole in half so you can slide it over the excess area. But try and find something (grease, saran rap) anything that the foam won’t stick to and cover the bottom of the plywood, then when you remove the plywood block, the foam will dry and not stick to or smear as you remove the plywood.
Geeze even I find that sentence hard to understand.