Managed another half hour today but posture was turning to agony so don't think it's right - it's improvised. I did have a zafu and zabuton for many years but they must have gone in one of my clearouts when I thought I was moving. I might look around for a mat of some kind and a little zafu, maybe make the mat double as a exercise mat as I need one for my stretches. Oh well, see if there's a cumulative benefit.