I'd go for design 3 made from solid blocks of steel- about 15mm wide then whatever dimensions you want for the triangle shape but I imagine about 50mm high and 50mm long.
from my experience with my flipper I managed to mangle a piece of 5mm aluminium like in design 1.
Also make sure the construction of the lifter arm is strong around the bends- mine kept bending made form 5mm aluminium with alu triangle welded on the inside of each bend as triangulation and it still bent! make sure when making the arm it was some material in a vertical orientation and some horizontal to resist bending- for example use steel angle.
Basically imagine what you would need in a worst case scenario and then triple it!