Grids are a popular studio tool for narrowing the light from a flash unit. During the past few years, these useful tools have made their way to the shoe mount flash community too.
Honl Photo Professional is one company making grids for shoe mount flashes. The company offers both a 1/8 and 1/4 “Speed” grid, each designed to narrow the beam of light your flash produces. Both units attach to a wide variety of strobes via a separate “Speed strap,” a Velcro strap that winds around your flash head and provides a Velcro surface to attach the grid to.
The Speed Grids narrow the light from your flash to a small circle (The size depends on distance from subject and which version you’re using. The 1/8 unit creates a smaller circle than the 1/4 version.)