I unironically think framing it like this makes it more visually and artistically interesting. Put a little title plaque underneath it -- "The Void" or something similar. If anyone asks, tell them you walked into the woods naked on a moonless night and ate the final piece.

Other options for the hole could include:

• A lock of your significant other's hair

• Your finger print

• A googly eye

• Pour black sesame seeds into the hole and fix them with glue

• A piece from another puzzle

• The words "You Are Here"

• A "peace" sign

• A picture of a slice of chocolate cake (e.g. "Piece of Cake" scratch-n-sniff sticker)

• This sticker about slots

• Wait until you have a bad scrape of about the right size, let it scab over, peel off the scab and seal it in resin

• "G ↔ ¬Prov(⌜G⌝)"

• A randomly chosen date sometime in the distant but not-too-distant future (when guests ask about the date, refuse to elaborate further)