I have
3 Slingshots. 2 cast iron and 1 aluminum. As a number of people have already said, the aluminum is too light. I ended up putting a plate of 1/2" steel under my aluminum to make it a usable weight.
Be aware, if you want to go the steel plate direction, you will need to fly cut the bottom of the rest to get all 3 corners even and level/parallel with the top of the center post. Then you can just bolt the plate to the rest.
FYI, if you have the option, get the slingshot rather than the triangular rest. The slingshot wil be a big advantage at those ranges where you have to crowd the outside of the bench, such as Kelbly's
David