You can order wood vent covers for forced air heating, made of the same wood as your floor, on the web or in advertisements in woodworking or homebuilding magazines. Other than that, any of the things you suggested are ok. You should use solid wood rather than MFD.

I made some for my last home out of wood. I tilted my table saw blade maybe 10-15 degrees, put the wood on it, raised the blade, lowered it, moved the fence over 1/4" and repeated. Keep the blade at a consistent height either by counting the revolutions on the hand wheel or minding the entry - exit points visually.

You need to use very straight-grained wood for this, or the vanes will tend to warp.

While this is not complicated, it requires some skill and safety awareness by an experienced operator.

"I believe both toekickes are of MDF material (right, not real wood), but I plan on lining the inside with aluminum foil. "

By the time you have enough hole area the remaining material is going to be very weak.

The first time a foot or mop bangs into it it is likely to simple fail.

Wooden rates need to be made of solid wood to have any strength.