I have thought about polystyrene pontoons a couple of times- based on the stuff industrial coolrooms are made from,
but before I could find any my lake dried up enough to use again

The depleted Gnangara water aquifier drains anything that falls into Lake Gnangara faily quickly, and is one of the few benefits resulting from over population and the water consumption in Perth outstripping the available supply.
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Just how deep does the water get at Lefroy?
Any fridge panels or foam offcuts in Kal?
Or how about stitch and glue flat bottom / open top plywood pontoons that can stack inside each other for transporting.
They could plug into the existing chassis rear axle sections, and have a cross support running from side to side at the front of the mini.
Either fiddle around with trying to use the front wheel for steering- or run cables to the back for a conventional rudder.
Whatever you try- you will need plenty of floatation as you dont want to drag your keels in the shallow water you have on lefroy.
Might be worth sweeping the concrete shed floor, parking the mini, and drawing some chalk lines to get ideas going!
stephen