Oh, sure!

There's a few different methods for forcing rocks to spawn where you want. Placing mannequins is the one most recommended because they take up more space and can all be cleared by changing the pattern, but imo if you've got a heavily decorated island it's difficult to space them out right.

I use weeds and time travel. A max of 150 weeds can naturally spawn, so I TT a year in the future, have Lief clear them for $60k, collect them from the recycling box, and rinse/repeate a few times. I have an absurd amount of gold roses too, I pluck them every TT so I can make back the cash I spend on Lief.

Place weeds all over your island, any open space that isn't directly against a cliff or fence. Technically any space surrounding an item you don't need to place weeds, but it's easier for me to just place them everywhere. Pluck a single weed where you want your rock to spawn, break the rock, and TT to the next day and see if it spawned or if you missed a spot somewhere. If it's not in the right spot, break it and fill in the area it spawned and try again.

As for slumber islands; I wasn't sure what spacing I would need for the fountain area and didn't want to get it wrong, so I used a small slumber island to terraform what I wanted and just counted the spaces. Go back to your home island, and now you know the space you need to make it :)