I gave my onions another haircut instead. The ones in the repurposed cardboard egg cartons are doing well but the ones in the actual seed starting tray keep dying off (I suspect root rot).Must remember that for next year.
I tried starting seeds in repurposed cardboard egg cartons one year and was not very successful. I think I overwatered them and they died. My fault, no doubt.
I switched to using 3-inch net pots with slits in them, and then bottom watering the pots in trays. That works better for me, and I have not drowned my plants since I started bottom watering.
Not only do the roots breath better in the net pots with slits, but the roots will actually air prune and new roots will grow, making the plant root ball stronger. No worries about the plant getting root bound with roots spiraling around the inside of a solid walled pot.