Thanks! I installed a 1/4-20 rivnut into the roof rib where the gray plastic tree holds the headliner. This hole is in the same location on all the ribs. Then I measured and drilled holes on the extrusion. I've thought about offering some plans for the complete build to make it easier for others.
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For my needs I decided the 16" was sufficient, I was able to get mine around $500. Tube length was standard 2x96" & 2x60" which was more than enough. I wouldn't buy their mounting brackets either as they're ridiculously expensive, although it is an odd mounting spacing you can easily make your own brackets with some aluminum plate or extra 8020 brackets, or try and drill straight through the extrusion.
Someone else on here found an electric version with 25" stroke for around $900 which would be a good option as well.