This topic has probably been covered previously, but a reminder is worthwhile. I have just completed an observer's chair using instructions off the Net (
http://www.jerryoltion.com/observing_chair.htm). It was very easy to make and works brilliantly.
Having had a leg amputated last year (due to cancer), I was desperate to find a safe, comfortable means to observe whilst seated. Tried the wheelchair, but this was obviously limited to a fixed height. This chair can be made in a few hours and only needs timber and screws from Bunnings, costing <$30.
It is very secure and does not have the collapse risk of some of the commercial alternatives, which mostly rely on friction. In any case, nobody seems to be currently stocking the commercial alternatives.