My alu mast is 1.6 mm thick in the wall and I use 6 monel rivets (4.9 mm) to secure the hound fitting. It is still upw there. And my rig is probably noticeable more powerful than your rig. I would take the plunch and use the 6 monel rivets. (NOT plain alu rivet NOT stainless steel, but Monel)

Wouter