I made one.
This makes me sound competent at DIY. I am not. It took me ages and ages to do because I am shit at such like things. However, if you or someone you know is competent, I think it can be done quite easily and rapidly.
I put gravel boards around my fruit bed to start. (I actually used the wooden fascias off my house, but you could use any wood of the right kind of size from gravel board to scaffolding board to stuff pulled out of a skip).
My neighbour was chucking out some hollow stainless steel poles so I grabbed these. But you could use any metal or wooden post. I drilled holes in the top at 90 degree angles from each other at the top and in the middle. I then postcreted these into the ground around 3 metres apart.
Then I got some plastic rope, and I tied it around the centre of the cage and around the top. I also put rope in a cross over the middle at the top.
I bought two pieces of inexpensive stretchy netting online. I used one for the top and one to go around the edge. I fastened these to both the poles and the ropes with cable ties. I made sure that on one side there was plenty of overlap, so I could make 'curtains' (like in a theatre, but out of netting).
I then put hooks into the gravel boards at the bottom of the bed and hooked the netting over.
The whole thing cost me about £25 for the netting, rope and ties and a bag of postcrete.