If you're willing to go aluminum either the zipp vuka 70 or 110 could be good options. I got my 110s for less than $70 on ebay and they're great.

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Cheapest hack fix option, find a piece pipe (pvc might work but I'd consider an aluminum pipe personally) as in insert in the area of the where the openings are for the cable routing/clamp area which will act as reinforcement to bear the loads better. You might have to sand it down to get it to fit though as it'd be pretty tough to find a pipe with the exact ID you'd want to press fit it in there (and if your extensions aren't smooth or inconsistent surface on the inside you might have fitment issues).

Another is to find the same style extensions made for clip on bars (that don't have any openings slotted in for cable routing for shifters). Then drill/file/grind out your own slots (if necessary) for cable routing.

Or make your own as there exists aluminum pipe in the exact diameter (or pretty darn close) and get them bent to your desired angle/dimensions. Somebody on here posted about doing a similar quasi-mantis position buying their bar (aka pipe) stock from McMaster-Carr and bending it.

You might see this thread....cyclenutnz posted a long list of extension dimensions...maybe you can find one that matches your test setup.

https://forum.slowtwitch.com/...ost=6792463#p6792463