Short-handed and rattled

Kovan, 17 km. A handlebar is meant to have two hands on it. But one of my hands holds a bar end - and a wheelset. The wheelset rattles. So I seek help from bikeshop man. He says something is trapped in the double-walled rim and I would have to shake it out. To console me for going home empty handed(!), he gives me a makeshift rim tape and tells me why the one I made didn't work well. As it turns out, even if I fix the rattle (I didn't), the wheelset doesn't quite fit (diameter is right, but not the width of the rim). What a difference 1-2 mm makes. What a difference a good fit makes. What sadness if something that used to go well goes awry or away ...