Ok let’s see if I can answer some of the questions.
I took two pieces of the flooring and glued and clamped them together using then tongue and grove. I had thought about trimming it and jut butting them together, but to test the strength of the glued t&g I made a small mock up. When I tried to break them apart, which took clamping them between two bricks and jumping on them the wood gave before the glue did. They are ¾” cherry hardwood.
I had thought of making the grills separate but again I made a mock up, testing spacing to aesthetics and again found the wood to be very strong. I wouldn’t suggest standing directly over the openings, but figured they are up next to a wall and had to withstand more bashing with a vacuum over someone standing on them.
I had thought of using a plunge router with edge guide, but after testing on the router table I found it easy to use. I just used a 3/8” up spiral bit, then attached scraps to set the length of the opening, and then clamped another block so the first slot was centered in the wood. I took between 2 and 3 passed per slot. I then had strips of wood that were the width of the space and slot and just continued to add a spacer, cut one side, then flip the wood cut the other, and just kept going until I had the number of slots I wanted.
When I rabbeted the edges I just made sure that there was between a 1/8 to ¼ of an inch of play. Figured one the vent openings aren’t all exactly the same size and again they should be in an area when they will be hit too often. They have two complete coats of poly, getting in the openings I found a sponge brush worked the best, and the top got a 3rd coat to match the sheen of the flooring.
Since this was my first attempt I can’t comment or durability or overall wood movement. I tried to account for most of this by leaving the center section close to ¾ “ thick and left the boarder 5/8” thick. As I said I did some testing and they appeared to have more strength then I had first thought. Was trying to give as much airflow, while keeping strength.