Step 2: Machining the mount
The basic idea is that you will have 4 tapped holes for the LED mounting screws to screw into. These holes are 34 mm apart on my LED plate; make sure to measure if yours are the same.
The first step is to square off your block. The dimensions aren't too critical as long as it's around 2x2x0.5". You need enough area for the LED plate to mount onto and enough thickness for the mounting screw to come in from the side.
I used the face mill available at TechShop San Jose for the facing step. It makes for a very clean surface. Ask the front desk to borrow it, and ask someone if you need help on using it.
The next step is to drill the holes. There should be 4 small diameter holes which you will tap out to the desired screw type. Make sure to pick the right drill size for the tapping step--look it up in a table if you need. Otherwise, you are very liable to break the tap. Tap each hole very carefully, since these will be fairly small holes.
The exact position of the holes is not too critical as long as they are exactly 34 mm from each other and square. You should definitely use the Digital Read Out (DRO) available on all TechShop mills for this part. And use lubricant! I almost had to start over when I broke a drill because the aluminum had fused to the bit. A little spritz of lubricant solves this.
After drilling the holes for the LED plate, you will need a mounting hole for the heatsink. There are two steps--use a drill, or even better, an end mill (to keep a flat bottom), to create a hole that the head of your center screw can go into. Don't drill all the way through--leave 1/8" or so of remaining material. Then, drill the rest of the way through with a drill just larger than your outer thread diameter. Done correctly, your center screw will slide in nicely with no part of the head above the surface, but with sufficient thread on the underside.
The last step is to drill the mounting hole. I used a 1/4-20 tap, as this is the same as the standard camera tripod mount. It gives reasonable flexibility in mounting choices. First, drill with the correct drill size all the way through to the center hole. Then tap all the way through. It's generally wise to tap in through-holes if you can as it allows the cut material to fall away more easily.
And that's it! You should end up with a block like shown in the last pictures.