You could throw a light sheet or something out there over them. I have containers on my balcony and when we've had sudden colder weather I've filled milk jugs with hot water and put them out, that's regulated the heat somewhat. But for frost I'd think a sheet would help.

Personally I know we've had frost while my alliums have been up and the've not been hurt, but I understand your concern. If I were in your situation and worried, I'd put something like a sheet or sheets of paper laid over top. Both of these are easy to collect the next day, are light enough to not damage the growth, and should offer some protection from frost.

Good luck!