50cm disc
I have 2 steel and one aluminium
Built a few different target designs over the years
I think the current one made from polypropylene plumbing supplies is the best
Lightweight, reasonably portable and uses camo netting for the main back and an aluminium disc Diana.
Also virtually indestructible.
Polypropylene is wonderful stuff
I just got the discs from a local aluminium engineering firm who owed me a few favours.
But they'd be cheap to buy
Just find an engineering firm and get them cut to size.
Dustbin lid sounds like it'll work well