Not as hard as it sounds - if the notch has reasonably square sides, then use it mark lines on a filler block, cut to size and glue up before tapping in. If the notch is not clean, then mark it with a square and undercut the sides a bit:
Original rough notch: _________ ___________ | | |_-_|
Undercut the sides, and flatten the bottom (couple of saw cuts down, and chisel and mallet for the bottom) _________ ___________ / \ /________\
(Multimaster type tools are good for this sort of thing...)
Use the outline to mark you block, and cut a plug:
______ / \ /________\
You can now slap it in from the side, and it will restore the compressive strength at the top of the joist, while not working loose, or pulling out of the hole next time you lift a board.
Yup that would do it as well.
Whatever you do, will probably take longer than you expect!