I made my own strap using bungee chord, velcro strap and some plastic boltsnaps. I also tried making a chin strap using ladies elastic hair bands.
Verdict? It's unnecessary.
This is what I do to secure the headband.
1. Put mask on and pull it down to your neck.
2. Put on GoPro head mount.
3. Put mask on, and pull mask strap over GoPro headmount.
It is extremely secure and if you accidentally remove your mask, the GoPro will not fly off (as it did when I used my home made strap).
I giant stride off dive boats with no problems, with my GoPro headband under the mask strap.
Best thing is, you don't have to carry anything extra, or have an additional piece of equipment to don/doff for each dive.