For those Do-It-Yourselfers out there, here is a description of the launch rail that I constructed. It will accept standard Series 1000 (BlackSky-compatible)  buttons.

To start, I purchased a 6' length of 80/20's "1010" extrusion from McMaster-Carr for just a little over $20 (part number 60585K41). I chose 6' because that's the length of my truck bed. You might want to go with an 8' section (part number 60585K61) for taller rockets or for slower lift-offs.  Keep in mind that anything over 8' will likely incur considerable "oversize" shipping fees via UPS, so taking the time to find a local 80/20 distributor would be worth the effort.

To attach the rail to the launch pad you'll need some sort of protruding rod. Originally I tried constructing a rod with a 1/4" threaded stud that could be screwed into the center hole of the rail. This had two disadvantages. One, I had to make different lengths of rod to fit various types of pads. Two, the 1/4" threaded rod bent quite easily no matter how tightly the rod was mated to the rail. In addition, it required the use of a tap and precision drilling techniques.

The second method I tried seems to be a winner. No special tools are required except maybe a drill-press, and the length of the mounting rod is easily adjustable. Figure 1 below shows the rail in its "storage & travel" configuration. Figure 2 shows the mounting rod fully extended.

I used a 1/2"-diamter rod because it's what I happened to have lying around, but you could probably use a larger diameter if your pad will accept it. The overall length isn't critical. Mine is about 19.5" (again, it's what I had laying around), but 12" would probably be more than sufficient.

First, drill a 1/4" hole about 1/2" from the top.  Then, drill a 2nd 1/4" hole about 1" more from the bottom than the deepest pad you'll expect to use.  I drilled mine 7" from the bottom.  These two steps are the toughest since it's important that both holes be centered and parallel to each other.

Now you need some way to attach the rod to the rail. Get yourself a couple of 1/4-20 x 3/4" hex-head bolts. The cap of each bolt will be inserted into one slot of the rail.  You may need to lightly file down two of the opposite ridges to get it to fit (as shown by the red arrows in Figure 3), but it doesn't take much... about 10 seconds worth in my case.

Once you've got the bolts in the rail and they slide freely, attach the mounting rod so that the protruding bolts go through the holes you drilled.  Add a star washer and wing-nut to each, and viola!  The wing-nuts can easily be loosened when you want to adjust the length of the mounting rod.

Now you just need rail buttons on your rockets and you're all set.  (Of course, those can be found at here  ;-)

(For the Series 1500 buttons the pad adapter would be a 3/4" or 1" rod and two 1/4-20 carriage bolts of appropriate length. The McMaster-Carr part number for an 8' length of 80/20 "1515" is 60585K63. The price is a little over $49. For some reason the shipping costs for the 8' of "1515" was quite a bit less than the 6' of "1010"... go figure.)

Figure 1

Figure 2

Figure 3