26 Proteus 3 months ago OR - sudden inspiration - fill it with concrete to stabilize the top! Easy fill through an f-hole. Stainless steel would be better - more sustain than concrete - but the molten form-filling steel would incinerate the wood and you'd have a blob of steel all over your kitchen counter.Be careful with expand-a-foam if you go that way...if you don't stop before it's completely filled, it will expand out the f-holes, and might even crack the top.Water would be interesting, if you could first plasti-coat the interior to keep the wood dry. Some caulking around pickup and controls, clear plexi panels inside the f-holes. Maybe some LEDs for mood lighting. You could keep fish - or at least line it with an underwater mural.If you wanted to keep it lighter, a helium bladder would work. I don't THINK it would be lighter than air, but maybe.Stay away from hydrogen. Hindenburg, etc.