Why not just use bear bangers? I forget the company that made the ones I used to use in the woods, but they were essentially a lower powered flashbang. Nowhere nearly as bad as a Tflash, but more than enough to give you a startle. They flashed a bit and everything. They're nice and small, and unlike those comical homemade grenades I've seen you don't need to tape a Bic lighter to your chest rig
Just take note of liability issues. If someone gets hurt, the user and the field owner/operator could be held liable for any damage to person or property.
I don't like using the real things even at work. I once got a front row seat to watch a professional doorkicker lose his entire hand to one. Picking up the pieces was pretty gross.