As Clint said, lower mass. I think that you want higher mass to dampen out any potential vibrations. Steel weighs 3x as much as aluminum per square inch.

I made mine out of 1" x 1" square steel. I bought it new at the steel yard and IIRC, I paid around $2 per foot.

I drilled the joints on a drill press and drilled some intermediate holes up to about 1/32 or 1/64th under the final hole. That way the final drilling operation is almost like a reaming and it is less likely to drill oversize. I wanted a close fit. Where the steel would rub against another piece of steel at a joint, I put in some HDPE "washers" and greased the holes to make the action really smooth. I also used shoulder bolts rather than have the threads working on the drilled hole. Additionally, I used Ni-loc nuts so that they don't self tighten our loosen.