How to make Explorer maps take you to an empty structure (1.21)

I'm making a skyblock map, and I want the player to be able to find ocean monuments, etc., but just the bounding box, not the actual blocks that normally spawn with the structure. The closest I've gotten so far is manually getting rid of the structure and creating my own that spawns guardians, etc., but then cartographers don't sell maps to it. I can't figure out how to just make the ocean monument itself spawn without any blocks inside, but in a way where you can still locate it using maps and use its bounding box to spawn guardians and such