I had a plunge bed built years ago with stainless steel. I went to a place that does stainless steel products for kitchens.
Its 3 x 7 feet six inches high, lip on top if folded over to prevent scarping and cutting my hands as I reach in. Put a drain spout on the lower part on one end in case I had to drain water out. Filled with a combination of gravel and sand. All kinds of things have popped in in it. The Othonnas seem to like it best.