[Porsche-O-Phile]

You could always give it to the guy as-is and if he fills it with water and blows it up after the trade, that's his problem.

If you're more ethical/nice than that, I'd look into constructing a base with an edge recessed a couple of inches at the perimeter of the tank, which will provide some lateral support. I'd also consider reconstructing the wood trim piece at the top to be a continuous wood piece (not jointed at the middle) with a "slot" routed into it the thickness of the glass, extending down a couple of inches at the top. Effectively this would act as a tension ring and help support the panels laterally at the top. With this, and the base at the bottom, I doubt it would go anywhere (assuming the glass itself is strong enough to hold and all the joints are sealed/butt-glazed properly).

Water is extremely heavy and exerts a lot of lateral (hydrostatic) pressure outwards on tanks and containers.