Like others said, this should be included with every head and only $39 if not less.  Haven't test it on a beauty dish as other have ... only on medium and large Chimera boxes and Profoto 1x4 strips.   All of us old film shooters were shocked the B1 and D1 flat glass panel was the ' standard ' configuration straight out of the box.  I finally got around to buying a dome and testing outputs with and without.  There is still no comparison when using this over a pure bare bulb head such Profoto's Pro B head or other manufactures like Speedotron.  With my Profoto B1, I measured a 1-1/3 of a stop difference between the center and far outer corners of their 1x4 strip ( and yes ... I had the baffle in there too ) without this frosted dome.  With the dome it's a little better but still around 8/10's of a  stop hotter in the center.

On my Speedotron and Pro B heads, I measured it around 4/10's of a stop ( a full stop difference better) !!! 

You can see how harsh the center hot spot is with the naked eye in the front panel with just the modeling light without the dome and a little better with.   This dome I would say is 1/2 as effective as using a pure bare bulb head.  I know some will say the B1 modeling light is back behind the tube so using the modeling light is not an actually representation of the tubes true spread across the panel.  Just test it yourself and you'll sell what I mean.  The distance between the speedring and outer panel on my large Chimera box is 30 inches so with the test image I attached here, I placed the D1's tip of the dome 30 inches from the wall.