As a result of the heating system upgrade, the ducts from the old system were removed, leaving holes in the wood floor in the previous supply and return register grille locations. The owner wanted to patch these holes with wood to match the existing floor which was not an easy task given that the original floor is over 100 years old, has been refinished several times and not perfectly level. He originally wanted a 45 degree bevel routered along the edge of the hole opening and then wanted the patch piece to have a matching 45 degree bevel so that it would be supported by the floor without any fasteners, just glue between the bevels. In a perfect situation this may have been possible, but the existing conditions precluded that from working. The existing holes had not been cut straight or square and the floor varied in level even over only three inches!
I used a circular saw, jigsaw and chisel to make the openings closer to square but perfection is beyond my current skillset. Even so, I would have had to make some sort of jig for the router that would have had to somehow been clamped or tack nailed or taped to the floor. I suggested to just use wood pieces with 90 degree edges that would be supported from below by ledger pieces screwed into the subfloor. That was deemed an inelegant solution since the patch pieces would not look great with the imperfections of the hole perimeter. More thought was put into other solutions including solid metal plate patches, new antique looking register grilles, wood register grilles, painting the old register grilles and hiring professional help.
The owner was unsatisfied with all of these options for various reasons. He wanted a solution that would be flush with the existing floor and not allow a collection of dirt to accumulate inside of or around the hole. When he wanted to insulate the floor (which is also the basement ceiling with the radiant system installed underneath), I told him that he needed to decide on a register solution first because it would be much more difficult to work with the floor once the insulation was installed. Under this pressure, he finally decided to go back to the idea of the wood floor pieces with 90 degree edges supported by ledger pieces. The wood he found matches that of the stairs they rebuilt which will darken over time and hopefully be a closer match to the old floor. I also suggested wood putty or caulk to fill the small gap between patch piece and existing floor to prevent dirt from entering that crack.