Can't tell from the angle of the photo but my similar cot is tough to assemble too. Then I realized that one of the end bars is longer than the other, use the longer one as a lever to pull the fabric tight after you attach the shorter one at the other end.
Does that make sense?
It's like there's a handle for you to get more pulling power to stretch the fabric with.
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JohnE
"and all the lousy little poets
comin round
tryin' to sound like Charlie Manson"
The Future/Leonard Cohen