nicely designed chain tensioner for bottom tubes ?
 

My bottom tube is 2 inches in diameter and I wish I could find a quality component that would raise the chain a bit to help clear obstructions and
minimize chain jumps off the timing sprockets.

Any supplier suggestions or ideas before I try to modify one from a motorcycle ?

Most (all?) high end tandems have an eccentric front bottom bracket to tension the timing chain. Lower end tandems often use an idler pulley on the boom tube. It sounds like you’re looking for a high end idler pulley. Check out the selection from T-Cycle.
https://t-cycle.com/collections/chai...on-idler-mount

https://t-cycle.com/collections/idle...ower-idler-kit

I'm not sure if I'm understanding your question. Are you looking to gain a considerable amount of chain to ground clearance below your timing chain? If you're looking to raise the timing chain by a marginal amount, one system that would likely help is one manufactured by da Vinci. Their timing rings consist of 34T rings, so they're considerably smaller in diameter compared to many other standard timing rings. It does require a square taper system, though. As noted above, the idler units appear to be a possible solution as well, but I can't say that I've ever tried them.

https://www.davincitandems.com/components/

We’re in our 31st year of riding a tandem and we have yet to have a timing chain “jump off” the timing chainrings.” Do you not have an eccentric bottom bracket to keep the proper chain tension? What tandem model do you own?