After several internet-inspired DIYs, ranging from not really practical to slightly less practical, I ended up with a piece of stainless screen which I found in a scrapyard, looks like a pump filter or similar.
Couple of 32mm stainless saddles to hold a garden hoe handle and it's an excellent bit of kit.
Strong enough to lever and lift reasonable rocks out of the sand, the wooden handle stays on the surface in the shallows without need for a tether or floats, the holes are smaller, but means I can catch earrings and chains which might otherwise drop through larger holes.
Only afterthought was to add some plastic hose over the base of the handle where I press the foot to dig in - after 18 months, it was starting to severely wear away the wooden handle
Mudwiggle attached the following image(s):