I'm trying to use the spawn on certain tiles one (using tilemap). Followed a guide on Youtube, which I think is similar to your idea (They record the occupied tile position to a dict and if there is already that tile in the dict, don't let the new resource spawn there), which worked nicely, but I had problem modifying it to work with larger-than-one-tile resources (for example, a tree might have width of 3 tiles and height of 2 tiles).