I recently had to replace my 30-year-old furnace and decided to update the floor vent covers too. (There’s really no connection between the two though since the furnace itself is hidden away.)
And it just so happened that a few weeks ago Ty Lam sent over some pictures of a set of register vents he made. I think they really look nice and they make for some nice household details.
There are a lot of different ways you can approach this project. Ty used a plunge router and a special jig to make the slots.