So it seems that a "par can light" is a light in a large metal can, used for stage lighting etc, with a parabolic reflector. (I guess an LED version wouldn't have a reflector, but would be in the same style.) And DMX is an RS-485-based serial protocol that's used to control stage lighting and similar performance-related equipment.
DMX requires a "smart" device, i.e. a microcontroller. So any project for a DMX-capable unit will have to include source code and a programmable device. It's not as simple as getting "some schematics". If there are no DIY projects for DMX equipment, that would be one reason. Another reason is that DMX equipment is normally used in professional applications where there is more money and less spare time available, so it makes sense to buy prebuilt products.
If you're familiar with DMX protocol and have experience programming microcontrollers, and you provide some specifications (starting with the details of the LEDs you want to use), we can help you with a design.