Proper method to install tie down anchors on roof?

Link to cleats: Amazon

You would want to use Dicor sealant under the cleats: Dicor

I'd use the largest self tapping screw that fits into the holes on the cleats. Maybe 1 1/2" long but you don't want to go thru the roof so that may be too long. Something like this: Screw

However my concern is just what do you plan to carry up there, how much does it weigh. That roof is not designed to be load bearing. And I have my doubts about how strong those cleats will be. Don't forget you are going to have the wind load to deal with besides the weight of whatever you carry. At 60 mph that wind load can be substantial.

If I was installing them on a boat I would drill thru the roof and put a metal backing plate in on the ceiling side with machine screws passing thru the roof. Not practical in this case.