I use the countersunk torx M6 security screw from Bunnings, only 304 not 316 stainless but I've had no problems at all. You would have to go to a specific hardware supplier for the cap screws but they would be difficult to find.
We have those item in stock its called Mh113 is m6 x 25 socket head cap with 2 washers they are $9.95 designed for kite fuelage 647, if its for a surf fuselage you need MH111 m6 x30 socket head cap with 2 washers again 9.95.