Well, the barn doors really do allow you a lot of control on where the light goes, and give you more options as to exactly where the light can be placed in relation to the subject. If you do not want the expense and weight of barn doors, you can also use slip-in pieces of lightweight, thin metal which slides into a gel holder. Like, on the small Photographer's Warehouse monolight 100ws Photographer's Warehouse PG3001MLB - photographerswarehouse.com linked to above, its gel holder will allow you to slide in a mylar or fiberglass or frosted glass diffuser AND also there's room to slide in different widths of light-blockers that will function like a set of 2-way barndoors.
If all you want is an overall, over-the entire set toplight as a hair light, the 30 degree grid on a 65 degree reflector is going to be about right for a 3-4 person group on a 9 foot seamless roll or muslin, with the light back near the paper roll. If however you want to use it as a single-sided rim light or accent light, on say 1 person standing, you will probably want a narrower swath of light, so the 2-way barn doors are ideal for closing down until only about 1 inch of light comes out and goes 10-12 feet and is about a foot wide, but tall. If you want the light to be wide, you rotate the barn doors, and presto! the highlight is all kept up high. Barn doors also quite often help keep your lens from getting a flare when the accent light is aimed basically back toward the people and camera...
So, if you don't have barn doors, you can use Cinefoil, or tin sheeting, or something else that's fire-proof,and use either gaffer's tape or a gel holder to hold some makeshift light limiters that do not weigh much and are basically free to make. As far as power goes: there is one weird,weird anomolous behavior of light,and it occurs when a hairlight/rim light is raked in from a steep angle,and that is this: the light becomes apparently "stronger" and "hotter" as it moves from the left or right side of the set, and around the back of the set,and the light meter's readings will easily fool you as to how hot the accent/rim/hairlighting will look, and consequently it takes VERY little light when the hair/accent/separation light is behind the subject. I read this odd truth a decade or more ago, and it was written by none other than CLTHRS...chief lighting technician, Herb Ritts Studios...perhaps *the* leading lighting expert in the field of celebrity portraiture in the world. Try it youyrself and see: with a separation light off to the side of the set it looks "one way"; as it is moved around and back of the subject, and as the angle becomes steeper, the light almost appears to "grow" in power, and hotness...it's a very weird and much unanticipated behavior, but CLTHRS tutelage is true...you do not need much light at all...so, if that 100 w/s monolight will go Full-Half-Quarter-Eighth
you ought to have 100-50-25-12.5 watt-seconds. I usually use 12.5 to 25 w-s for mine, with a 200 to 400 mainlight.
