I owned a JS1c 18/21 in a standard 15m type Cobra trailer and it was just OK for width except that I had to be careful to lower the top exactly centrally otherwise the flaperon horns at the rear of the trailer could get caught (never did). The problem for me was the lack of adequate storage space at the front of the trailer. A better option would be the same width but 40cm longer part no 031/030 trailer which also has a slightly taller top. I have helped to put a JS1 in one of those different and it makes more of a difference than it sounds. I am going to get that size for an 18m-only glider just for the extra storage.

I have also seen a JS1 in the Cobra "DoSi" Duo type trailer and it certainly was extremely roomy but it looked like overkill to me.