Looking for a knot that can be tensioned.

Hell awtchi and welcome.

I think before a real meaningful answer can be given you should provide us with more information. What is the purpose of this construct? Will the rope just be a guide line? Will it have something else attached to it? Will this be a permanent thing?
How many poles in all?

Your poles are not all that substantial and your rope/cord is very substantial. I would think that any tension between the poles will bow them toward each other and you will be adjusting the tension till they touch!

I would not go so far as to say, as you have, that the fixed end is auto explanatory. You show an eye knot, but this can slide up or down and might need to be a tight hitch instead. Or the rod(s) may need a through hole(s).

We can go from here with some more data.

SS