I bought some 3/4"x1/8" flat stock from Lowes.  It was appx. $3.50 for a 4 foot piece and $2.93 for a 3 foot piece.  You need (2) 4 foot lengths and (2) 3 foot lengths.  I choose to use 1/8"x1/4" aluminum pop rivets to fasten the frame pieces together.

I started by laying out the cut flat stock pieces on top of the old frame.  The old frame was tin and had rolled edges and was welded to each other.

You could weld the flat stock but since I don't have a welder or torch, I choose the pop rivets.  Since my old frame failed because of lack of support and metal fatigue, I choose to use three short center supports, vice two.

I drilled each piece of flat stock on a drill press with a 1/8" drill bit.