Probably wouldn't matter. There are some reflections that escape from the top and bottom (some light reflects off the Fresnel lens then bounces back forward off the flash head) but that light would be lost anyway. Maybe you'd get a little more power if you lined the 'arms' and added top and bottom pieces with reflective material, but since the arms are black… Also, the idea is that the zoom setting on the flash head is appropriate to cover the Fresnel lens and not much more than that. I haven't done the math, and it's likely going to be different for each flash head, but I shot the projection of the bare 600EX-Rt at different zoom settings from a distance of 5 feet, with the dots on the wall spaced one foot apart. Some geometry would let you calculate the coverage area at the distance of the Fresnel lens from the flash head.



Side note on the Better Beamer… It's sometimes called the Better Burner, for good reason - a Fresnel lens works both ways, so if you're using it outdoors in sunlight, the sun shining through the lens can cause damage to your gear of the 'hotspot' falls on your flash, lens or body.