Though I haven't worked with those halogen work lights, I am very familiar with open faced tungsten sources. While you can spot and flood some open face units, the ones you're working with usually don't offer that option, which is one reason why they're $25.
The cheapest and easiest way to narrow the beam spread is to use blackwrap on the unit, only be careful to allow a little breathing room so they don't overheat and blow the bulb. If you put gels or diffusion too close to any light, it will burn up and melt. Diffusion should be put on frames away from the face of the light. If you put it right on the barn doors it isn't really softening as much as it is just cutting the amount of light. The further away you can get it from the source, the softer the light will become.
One effective way to turn those worklights into something useful is to bounce them into bead board and let the bounced light pass through diffusion, this practice is commonly called a "booklight". It's useful for creating soft light in close courters where the units are fairly close to the subjects. The other way, if you can get the lights far away enough is to aim them directly at the subjects and sets but make sure you have 6x6 or 4x4 frames of heavy diffusion. 216 or greater at least a few feet away from the light.