Laid out a hole pattern in the 22" long top channel. 9 rivets per 11" section, about 1 1/4" apart, with 1/2" on each end. Drilled this 3/8" from the edge, since the channel was 3/4" deep. Marked a line 1/4" from the edge of the side pieces, and drilled these to the center piece. Figured out what other pieces of .125 are needed to put a little doubler at each end, and join the side segments in the center of the 1/2" flange. That way, if anything heavy drops on this cover, it will be more resistant to bending.
