Columbus make a couple of 42mm down tubes including one from the 29'er tubeset in 750mm length (which is probably half as long as you'd need)
My DZB tube was only available up to around 750mm on the Talbot and barely made it. The Isen was able to use it as the Pinion plate extended the reach
You could go for Killer V style, just upside down. Downtube joined to the top tube with a support tube to the headtube
@BareNecessities started
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Columbus make a couple of 42mm down tubes including one from the 29'er tubeset in 750mm length (which is probably half as long as you'd need)