Hi, yeah, I've designed a few somewhat crude projectors for various uses. The way you do things sort of depends on exactly what you want to do with the light, and how much money and effort you are willing to put into it.
You mentioned using a slide projector, but mounting it, etc., is a problem. Well, I would go along with AgX; it's easier to build a mounting fixture for the slide projector than to rig up your own projector. If necessary, pull the guts out and remount on your own baseplate, or whatever.
The method I think you are considering is to use your own projection lens with a focused "slide," perhaps just a metal plate with a hole cut in it, placed in front of one of your light heads. The problems with this are that 1) it's very inefficient, perhaps only 1 or 2% of the light will be used, and 2) the light will not be even. You could probably even it out with some diffusion material behind the slide, but then you lose more light.
What you would want to do with your system is probably to build in a "condenser system" behind the "slide." This would essentially be a lens system that projects an image of the light source into the aperture of the projection lens (note that any light NOT entering that aperture is lost). This will automatically even out the light as well as greatly increase collection efficiency. As a rough guess you might get 10 or 15% of the light.
So the slide projector is starting to sound pretty good, huh?
I'll be glad to explain further, or kibitz on your design should you want to go that way.