Small Tunnel Shapes

I'm in the market for some new anti-insect mesh to protect my alliums from the leaf miner and brassicas from flea beetle and as my hoops made from left over rebar aren't really big enough I though I'd invest in some supports too. But why is everything round, meaning that the outside edges can't be easily used?
Does anybody make/sell rectangular frames to support mesh or plastic? I can't find anything on the French market, beyond modular solid plastic cloches.
I may have to set MrPP a task, although he still hasn't made the bench from the tree my neighbour cut down three years ago ...