The strap you suggest already is on the market, made by Niteize. It works, depending upon how yu place it on your head with any light that will fit within its two straps, basically either a AA tube or AAA tube.

I don't regard the H10 as too expensive. It is a quality, dependable light. I willingly pay more for more powerful, versatile lights

In many emergency situations, your light,usualy a headlamp, becomes a crucial gear item. Ask any caver....