You can buy a tarp garage to cover a small truck from Machine Mart for £360. Then there are Party Tents, available in a large range of sizes, and standard, heavy duty and extra heavy duty gauges. A 6 x 4 metre tent would be around £400, perhaps £650 for the heavy duty.
Then I saw this rather beautiful structure at a music festival, and thought: I could make one of these instead.
It's a 2V dome, very simple to make. What I like about this one is the way it's been covered with one large tarp and one small one. The large tarp has extra eyelets along one edge, and these are attached to struts along the opening. Then the whole sheet is pulled taught from the back and tucked under.
The dome pictured is around 4.5 metres diameter. I will make mine a little larger - perhaps 6.5 or 7 metres. This will mean that the "door" (as shown above) will be an equilateral triangle with 2.1 metre sides.
I thought about making it from hazel rods with water pipe strut endings, but had some anxieties on a number of fronts: would I be able to find a source of straight hazel rods, 2.1 metres long? Would any knobbly bits on the hazel rub holes in the tarp? Would it be strong enough?
I have decided to go with "Cundy poles". These are softwood poles, possibly forestry thinnings, that have had the bark removed my machine. The diameter is variable, and they are not absolutely straight, but they look strong, durable (they are generally used as stakes in the ground) and smooth enough to support a tarp without damaging it. They are also much cheaper than machine-round poles.
I've bought 80 poles of 2.4 metre length, now stacked up wigwam-style under a tree: