In this video I show how I mount my windsurfing board and masts to the crossbars on a Tesla Model 3. This technique also works with SUP boards and paddles.
Your straps will damage your board rail like that. it needs to be placed bottom down so that the straps go with the curve of your deck. So backwards with fin down.
Interesting suggestion, but I haven't damaged it yet and I do feel better following the conventional wisdom. Folks around here say that bottom up makes the wind push downwards rather than upwards, which helps prevent flyoffs. I suspect it doesn't really matter much if you've strapped it properly!
Why don't you put the strap through the carry handles of the mast sleeves? It would 100% prevent them sliding out from the straps and doesn't add any trouble.
