It’s far more likely to break from thermal shock than physical trauma. If the HA filter gets too hot it can break. Any blow to the head strong enough to damage the HA filter should be obvious to the outside of the unit.

The Surplus Shed has small HA glass filters that are reasonably priced. Type “heat absorbing glass” into the subject line to see what’s available.

http://www.surplusshed.com/search.cfm

You could also buy a larger HA glass for a Beseler or Omega enlarger on eBay fairly cheaply and cut what you need. These show up often for modest prices. The Beseler HA glass filters I've measured are 1/8” in thickness.