On Tuesday, July 18, 2017 at 7:19:51 AM UTC-4, Trumble wrote:

I want to put a 304 stainless cap on the end of the tube capable of
handling say 20 kg pulling force.

When fitted to the end of the tube the gap from the cap to the
polyurethane potting material will be 1/32"


I think silver brazing would cause much less heat to the polyurethane. But think it would still be too much heat.

Think about why you want the cap on the end of the tube. If it is to prevent liquid from reaching the electronics , you might find a O-ring seal would work well.

Dan