@Dannakyri1 I love this idea!
Sadly, I don’t think there is a way to force an urn to appear as a tombstone inside without mods.
However, I’m just thinking outside the box a bit but an alternative would be to use the generic tombstones available in debug mode and have the actual interactable urn at the foot of the fake tombstone. You could use the reduce size and move objects cheats to make it look like a small flower urn placed on each stone. It’s not ideal I know :(
If anyone has any better solutions, I’d love to know too, as I’d also like to make a similar crypt.