Yep, metal studs without wood blocking are basically worthless for holding anything besides sheetrock.

A couple of other options that I've used are through-bolting to another 1x4 on the other side of the wall (depending on what's on the other side) or using vertical legs down to the floor. You might also try something wider than 1x4 so you could put 3-4 screws into each stud. Make sure to use screws specifically designed for metal studs (fine, not coarse, thread, and drill point).
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