Just hang it secure the air movement can move them around a little. Are you using the silver flexable dryer vent hose to feed the air in to the light?

The little shaking can reduce the life of the bulb by a little but 400w is not that expensive and they even sell them at HD.

You can cover the crack with foil tape but it can contuine to run. The gule used to fix windshields will not last long on cool tubes they get to hot. btw