So I maybe getting one of these

LOL, you serious? Haven't you made tensioners before? weld a washer to a 6mm tapered solid tube, cut a square rod down to about 40mm and machine it out and drill a 7mm whole and then buy yourself 2 bolts.

Very serious and rightly so.
you need something like this.
http://www.bikedock.com/posit/shop/search.php?searchtext=99085469&format=1
try UTFS it's a really clever idea.
https://www.londonfgss.com/thread19372.html

Easy.

Which one are you?