How to simulate fresnel lamps I am doing a theatre scene, which is mainly lit with fresnel lanterns (https://en.wikipedia.org/wiki/Fresnel_lantern). The lamps themselves are not visible, but their light cone is. 

Unfortunately, I can not get this to look right with blender. First of all, spots seem to be incapable of other forms than round, but due to the use of barn doors (https://en.wikipedia.org/wiki/Stage_lighting_accessories#Barn_doors), theatrical lighting equipment gives more of a rectangular shape. 
I fixed this with placing my spot inside a cube which is open on one side, however, this gives me rectangular cones with sharp borders, rather than soft borders as real fresnel lamps do. 

Any hints on how to get a rectangular cone with soft borders? Ideally, it would not be too hard to change angle and size of the cone, as I may need several attempts to point them correctly. 

I would prefer a solution that works with Eevee, but I can switch back to Cycles if this makes things easier.
EEVEE:

This is as soft as you could possibly get with EEVEE.
Unfortunately softness is &quot;hard coded&quot; so you can't change lamp distance to have more/less softness.
I have used the same method as you - Spot Lamp enclosed in a Cube. I've also parented Lamp to the Cube just to make ability to move/rotate them nicely.
Lamp settings:
Basically this are the only settings that have influence over the lamp softness. Not much to play with:

Softness to the max - 100, Bias to the minimum - 0.001 and Exponent is a matter of taste.
There is not much to explain here, but if you have more questions feel free to ask in comments.