Fortunately, in my training, I was able to assist another club member in the assembly of his private glider.

Careful, 61.87i-13 says the glider, not a glider. Just saying.

Thinking back, by the time I soled, we had done an annual on the glider in question, which sorted out assembly.

Given a choice between a 'performed 61.87i' solo signoff checking everything on the checklist and thinking if the overall situation is a good idea, choosing both sounds good.