I am always wary of drying onions in the greenhouse as it can get so hot in there. John had an experienced which resulted in a cooked onion smell too, so I am now doubly wary
The stems only need to be long if you are going to string them up, so any shorter ones or ones with fat necks you can just use first.
I stored mine in those heavyweight cardboard box-trays that fruit comes in in the supermarket last year - they stack well too - and this worked well
(strings snapped the previous year with the weight of onions so gave up on this, even though they look brilliant)
Nice racks there Gavin