A thumbtack or screw on the top of the blast button would keep you from completely covering the opening when firing the hornet tank and causing a misfire.
He has a thumbtack
Now, take your thumbtack and heat it up while holding it with pliers. Melt it in the button and you're good to go. http://i339.photobuc.../NonNerf212.jpg
Couldn't you just take a heat gun to some CPVC and bend it so it fits in the shell? This would remove the need of an elbow and cut down on the dead space.
in theory, this could work, especially since the cpvc wouldn't be operating in a pressurised capacity (ie as a tank), so you wouldn't need to worry about compromising it's integrity. However, bending pipes with heat guns is iffy at best, and bending them to the tolerances needed to both fit the shell and achieve a tight seal to the turret... well, I wouldn't try it, but you're welcome to!