I don't know if anyone will read this or do it but here goes:
I have made them with a friend before and they came out great. The parking block flatbar at the Oakland DIY spot everyone was skating for a while was made with them. It is pretty easy. Before I explain the design I will say all the surfaces on the inside that will actually form the parking block should be lined with trash bags. This will give them a sheen that is really smooth on the outside. Take a 2 x 6 and nail 2 x 4s on top of the 2 x 6 (so it looks like a bar of staples). Then cut the edge off two more 2 x 4s at a 45 degree angle and screw those onto the top of the 2 x 4s that you originally attached to the 2 x 6. And lastly you cut out end caps (of course you can make them at any length). This will create a form of a parking block with an open top. You need to make sure your structure is sturdy because a lot of concrete is going inside. Start packing cement inside (and I mean pack it in. Pound it with wood or trowels). Fit 2 pieces of rebar inside at some point. Really pack it down until you fill it and then trowel the exposed surface and seal it with another 2 x 6 with the edges cut at a 45 on each side so it fits together (will of course have to trim it down a little. A 2 x 4 would be too small for this purpose). Let it sit for a day, unwrap it and then let it dry as slowly as possible by keeping it as wet as you can (poor water on it a few times that day).
Not sure if that is decipherable, but it is easier than it sounds. We made 3 four foot parking blocks in a few hours and it came out better than any parking block flatbar I have skated. And to actually make the parking block flatbar we just used liquid nails. It is easy, quick and was totally durable (that barking block flatbar was skated a lot).