Not haveing started one of those Chinese in frame projects yet I've been following these feedback post on the issues builders are having them. The thought entered my mind for a spring tensioner similar to the rear derailer tensioner (although I doubt one would be available for a size #41 chain)
But in principle....at least it would take up the slack and be flexible.

It seems these chain tensioners are really the #1 issue, followed by the rear sprocket mounting.

I'm sure that in China were they are cranking out this #41 chain that they could easily tool up to make 1/2 links and in time a better tensioner. Usually when there is a crying need for something better someone rises up with "aftermarket" solutions like we're seeing here with 99's solution.

I'd like to see 99 take his photos and put up a post on the "Take a tip & Leave a tip" topic. If he wanted I'd be willing to sketch a black and white projection drawing of how to do it for those wanting to try it.

.......and Egor, thanks bunches for explaining that chain geometry. Recently I was reading about a "lock up" condition where the chain kinking up in the counter shaft gear housing could lead to that...........which once again is a chain tension issue. I've wondered when I look at alot of those old turn of the centruy British bikes that had "chain guides" (I think I saw one set up on an old Excelcior bike) before rollers became popular that just setting up a guide may be the way to go. Yeah, it would be cheap & dirty.....and likely loud....but perhaps dependable.