[lastlight]

Oh now I see the pic. Lucky you it's not actually moved apart... I would totally have the exact piece cut and cover the thing with silicone and toss it on top. No need to disturb the tank either! The curing fumes should just dissipate up and out? With the entire thing smeared with silicone and pressed together the two pieces wouldn't budge at all if the bottom one ever cracked I don't think (massive surface area contact).

I'd never clamp a tank that's under load but that's just me.