Stainless machine screws with a pan head (I believe 3 1/2 "), fender washers and lock nuts for the underside.
Get a sheet of neoprene rubber about 1/4" thick to go between the t/m and the boat. This allows for compression when nuts are tightened that reduces shock and helps keep things tight. Better than the individual rubber mounts in that it contacts the entire surface between the mount and the boat, not just 4 small corner areas.
I get mine at Texas Rubber Supply in Dallas. Texasrubbersupply.com
I'll never mount a t/m without it.
God does not charge time spent fishing against a man's allotted life span. - Indian Proverb