It doesn't really matter what light you use, but you should already understand if you want an edgy look you shoot with a smaller light. You want a softer look, you shoot with a bigger light source. This is really quite easy.
Get your self a scrim--build it, buy it, find it--our of wahtever material yuo want. Since it is going between the light and subject, you can make it out of cardboard for ease.
Put the light behind it.
Adjust the light and or scrim up or down to get the angle you want and the right width for your fake frame reflection.
Lighting: Just play with it. Use an umbrella and move it and the scrim you made closer or further away for a softer or harder edged look. Yo don't need any fancy this or that. If you want to concentrate the light going through your "window" wrap some cine foil around the reflector to form a large opening snoot to throw the light in a tighter circle. This means you would be shooting bare bulb, w/o any light modifier, less the stock reflector.
After you get that set up, set you main light as you want it, then reset the fake shadow to an intensity you want for the shadow effect.
There is no way to know the exact distance of the fake window sill you will make or the fake branches either. Just move them around and you'll get it fairly quickly, like about 5 shots using flash.
Flash-->fake branches-->fake window frame-->subject
Just move them around until you get the effect you want.
Am I missing your point?