Best reflector type for rx7 lamp fot scrog

Hey , i want to build my own reflector for a rx7 (double ended ) type lamp , my scrog area will be about 18" in width and 20" in length .

I have not used these style lamps before and would imagine they would not spread the light past the end caps? And only been 6" or so long it seems as if they would throw more of a long rectangular print ?
I have been thinking maybe an octagon style reflector to catch the light and hopefully distribute the light in a more circular foot print .

I have my gear to make it , and found plans for a few diy reflectors but cannot decide !

Any help is appreciated

Oh and by the way its ge 150 watt cmh bulb , forgot to mention that . Cheers