Good chair for stretchsitting?

I'm Australian, so can't get the Gokhale chair. Currently have a secret lab chair which is absolutely awful for posture and doesn't work for stretchsitting. Also have no seat at home available to me that I could modify - they are all designed to force you into bad posture. I've looked at Officeworks for some chairs but all seem to either have contoured / concave backs and seats, or too much cushioning / lack a firm backrest. The folding chairs for example have contoured backs and also have a posteriorly tilted seat which raises the knees over the hips.

Of course, I could just buy a stool, but I have heard that stacksitting is not recommended until you have eliminated any swayback / lordosis - which I have. I previously thought it was anterior pelvic tilt, and constantly tried to tuck my pelvis and flex my abs hard to "normalise" my posture, but after discovering Gokhale I realise my problem lay further up in the lumbar spine. My back sways and my ribs flare out (more on one side might I add), making it look like I have a belly despite me being lean enough to have a six pack.

Does anyone know of any good chairs for stretchsitting, preferably in Australia? Willing to buy online if necessary. Any recommendations would be greatly appreciated. Thanks.