Y'know...it's not so much the knowledge as it is the tools and the certification. The products we use aren't exactly difficult to create - but building one...that's different.
List of item you'll need are:
* Supply of LEDs in the proper color temp and amount for your application
* Power supply and circuit to drive the LED
* AC cable not exceeding 12' in length and typed to match load
* PC board of a design to adequately mount the LEDs
* Case, constructed of metal or thermoplactic
* Yoke or other mounting device (third party supplied or design and mill yourself)
* Misc. hardware (screws, bolt nut washer, grommets, etc.)
* Drill press and jigs
* Metal saw
* Metal bending tool(unless you buy case from a third party)
* Soldering tool and heat sink - LED can be dammaged by heat like any diode
* Buffer and grinding wheel for deburring and polishing
IF YOU USE PLASTIC:
* Thermoplastic peg table or vaccumn table
* Forming tools
Note the Drill press, bits, saws, buffer and grinding wheel can be used as well - but you'll need specific wheels, bits and blades for plastic.
Once you have created your design, it's time to prototype. If you are using this for just yourself, then your prototype becomes the end product or possibly first of several you'll build for your use.
You'll also want to register your plans with the Patent Office...just in case you've, A) infringed on someone, and, B) might have designed something unique - you may not want to manufacture it, but you don't want someone else profiting off your hard work eiather.
Last is getting certification. If you use the device at home you have very limited liability - if you use it for hire there will be insurance issues - most insurance riders will demand UL, CSA type acceptance, many will accept Calif. Fire approval. If your "homemade" device lights up a fire you will automatically be liable - regardless of how professional the construction was.
This isn't to discourage you from building your own devices - I've built many in my shop. But I did spend time at Century-Strand as an assembler and machinist (thought it would be a stepping stone into lighting design - it wasn't) and know the complexity of the build process and the legalities.
Bottom line - unless you are developing something for retail, the cost of one-offing is far more (labor, supplies-parts, etc) than the cost of purchasing.