You need a brolly first
A sorta nut on a plate.
If you use this method you will have to beef up the area, on the seat, adjacent to the bolt holes because there are so few areas taking all the load.
Sikaflex is the product. Unlike silicone sealer, it will go off in the tube once opened. Don't expect to use half of it and go back in a month to use the rest.
If using self tappers:
1) don't drill right through the deck.
2) do drill the correct size of pilot hole.
3) don't overtighten the screw.
So many folk tighten until they feel a little bit of give. At that point the thread has just been broken. Remember, with any screw or bolt. Tighten to hold the article in place only. The load will further stress it. It is no use being on the brink of maximum strength and then expecting the thing to take a load also. Common sense you would think. But....