So we fashioned some fixings from stainless steel, which was cut to size, shined up with a grinder, and bent to shape.
The underside of the seats was 'dotted' with Sikaflex to create a rubbery base so there's no chafing of the wood on the IBC. (The Sikaflex won't actually adhere to the IBC material).
The steel was slotted under the top of the hot tub, so they remain removable for cleaning, but we have no issues with us floating away!