I settled on a design and we have almost finished splitting all of the Sonotubes. The jig I ended up making was a 16' long piece of plywood 18" wide as the jig base. Onto this was attached a movable "fence" that also went the 16' length. For the 16" diameter tubes, the jig fence was placed 8" from the edge of the jig base. The tubes were clamped to the jig fence at both ends, then the jig was run through the tablesaw with the saw fence set at 18". This gave us very straight, quick cuts. Then we rolled the tube 180 degrees, used a framing square to confirm that the first cut was rotated directly above the next cut. Clamped again and ran through the second cut. I used a feather board to keep the jig against the saw fence and roller stands on the infeed and outfeed to support the jig.
Thanks for everyone's input!
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