I second that it would not be very likely that you could have such parts custom made for less than FourTrek's price. They are set up specifically to make those parts, they already have a tooling advantage over anyone else.
There is, however, a product known as a "split shaft clamp" or a "split collar clamp" that is available from Reid Tool or McMaster (to name a couple of the likely many vendors) that is essentially the base part of the FourTreks design. The major difference between them is that the FourTreks design has a much larger OD, which allows for much easier attachment of the secondary brackets.
Similar to Moody's idea, what I've seen done is to use a piece of 1/2" round bar stock with a hole drilled crossways in one end, & welded to whatever with the cross-hole oriented length-wise to the jack's beam. Then slide a shock bushing down over the bar followed by the jack's beam, a washer, and then insert a hat pin in the hole. If you set the bar length correctly there will be a little bit of tension on the hat pin. Pushing down/in on the jack beam allows removal of the hat pin & washer.