Drying Onions

As long as both ends are open for circulation you'll be fine drying them in a tunnel. Or if one end doesn't open, put an electric fan in that will move the air and keep the door open. Same with sheds.

I've never found any difference in hung or laid down techniques. Do whatever you can with what you have available.

We generally dry ours out in the greenhouse, not had any problem with them cooking so far 

It's a 'second' greenhouse though, with nothing growing in it at the minute, so the door has been open for good air circulation. I'll probably take them into the shed in a day or two though, as I store them hanging leaves down on a rack. I can't seem to store them at home - perhaps the central heating is too much for them? - so the unheated but well-insulated shed seems to do the trick. We did lose a couple to the -10 degree frosts last winter, but that wasn't exactly an average winter 

Should be fine