Copper Pipe sizing for the box collector

I was thinking about starting to make the box collector today, and was looking over the parts list, and I see a couple of things that I have questions about.

Copper Pipe - 22mm - I've never seen that in California - I think we pretty much have 3/4inch and 1 inch - 22mm = .866 inches, so The pipe I know I can get will either be slightly bigger or slightly smaller - I'm thinking about the 1 inch pipe so that the tensioner will fit easily - any other considerations?

I may have to mash the whole thing more in order to decrease the volume - how essential is the volume accuracy?

I see also that an m6 nut is a metric nut - we'll see if my local hardware store has them, and it looks like a quick search for a dunlop valve doesn't turn up much, so I'll see if my local bike shop can get one.

I was thinking about starting to make the box collector today

Nice. Glad to hear it.
I see you're also interested in solar cooking. I suspect the box would make a pretty good oven, as it has a fairly high degree of concentration and about a 2.5 hour range, so you wouldn't need to reorient so often.

Copper Pipe - 22mm - I've never seen that in California

It doesn't need to be precise, just use whatever's closest. The size of the reflector is set by the width of the flattened pipe, so if you end up using something that's 10% bigger than the 22mm, you can also scale up everything else by 10%. But you don't have to.
As long as the box is the right shape, and the front edge of the copper is sitting at the focus as marked on the templates.

how essential is the volume accuracy?

Not. It works out that under general use 10ml of ethanol more or less empties a 400ml tin. You want both running out around the same time, but it neither needs to or can be particularly accurate.

I see also that an m6 nut is a metric nut

Whatever fits. If you get the cable tensioner bolt off a bike it should have a nut on it, and the valve has a weird thread anyway, something imperial can no doubt be made to work.

it looks like a quick search for a dunlop valve doesn't turn up much, so I'll see if my local bike shop can get one.

Yeah they're not common in the States. They can be bought online, and given how completely perfect they are for this, it's worth the effort.

Good luck, do post how it all turns out for the benefit of others not in the metric world.