My Topeak rail arrived the other day. Here is my method for attaching the Topeak adapter rail to the rack on my RadCity ST.
I started with 2 of the smallest EMT brackets which you can pick up at your local hardware store. I used some Romex insulation as the rubber pad between the bracket and the rack. I had some scrap aluminum that’s about 1/8 inch thick that I cut into 5/8 in x 3 in strips to be used as a plate to bridge the ridges underneath the Topeak adapter. I drilled a hole in the middle of the plate the same size as the screw. I used the original screw and a locknut to secure it to the frame.
The front EMT clamp needs to be modified slightly. I had to cut about 3/16 of an inch off of the end of the EMT bracket that curves around the rack pipe. This pipe is smaller than the rear pipe we will use which also requires that you bend the clamp a little to make it fit tighter around the pipe. Cut a piece of the Romex insulation to fit the interior of the bracket. Clip the bracket to the underside of the rack crossmember with the hole pointing to the rear of the bike. Place the plate on top of the EMT bracket aligning the holes and then the Topeak rail adapter. Insert the screw and secure with the locknut. I won’t kid you - it takes a little fiddling to get everything to line up but you can do it. A little longer screw would have made the job easier.
The rear bracket needs to have it’s hole elongated in order to allow the screw to pass through as things don’t naturally line up. Since this crossmember of the Rads rack is larger, no need to adjust the width of the clamp. You may need to elongate the hole in the aluminum plate as well. Add a piece of Romex insulation and attach. The last three pictures show the rack mounted from the top, clamps underneath and the bag attached.
Hope this helps some of you out there looking at the more expensive options. $18 for the Topeak adapter and ~$1.50 for the rest of the parts. Chances are you can get the EMT clamps for free if you know an electrician.