Our client is a keen archer to competition level and currently competes from a wheelchair, but as he still has some mobility in his legs, he wanted to compete in a different (seated) class.
The current world champion has a custom seat made by Loughborough University, and our solution for our client borrows some of the principles - namely suitability for use on grass or indoor surfaces, and guides to ensure the feet are positioned correctly to meet competition rules.   
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The current world champion's stool
These competition rules are summarised below:
We identified a simpler solution which was a neat five castor gas-lift therapy stool which would fit within the required footprint (shown right) and cost about £60!
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A strong timber base made from 3" x 2" studding, and with captive ground spikes, allows the therapy stool to be used on grass with the castors kept clear of the ground.  Three trimmed off pipe clips locate the base of the stool so it can be easily lifted off.
Polypropylene strips were then heated with a paint stripping gun and bent to shape, then allowed to cool in the jig as shown.
These will become the leg guides to ensure the client's feet sit within the competition footprint.
The final solution showing the grass base and leg guide in place, and below, testing.
Harrogate and Ripon Group