Two bungees grip the sliding extension, which is a 24" piece of the same 2x2" stock. This is plenty holding power for the lightweight buddipole antenna (configured as a horizontal L here).
I'm thinking about modding this to support a turtledog boom on one end, with a variable height adjustment.
They make metal devices*1 to do the same job as the bungees, allowing a remodeler to jack up a porch with a failing upright, when replacing it with a new one.
Two straps (here, bungees) act as a kind of sling; the sliding and stationary upright could both have notches to accept what amounts to a 'bail' in this jack system.
The bails are generally at a small angle to the perpendicular of the major axis of the two members they bind together, and work due to the locking effect, and only when vertical (in gravity).
You could maybe compress the ends and pin them as well, to force the bails to hold fast and take up the dimension between the two pieces of lumber (at the friction/bearing surfaces).
I use zip ties for the bails, in lightweight applications such as this. The bungees were an interim step, used during prototyping.
The topmost bungee does double-duty in both supporting the sliding extension, and fixing the apex joint (of all three uprights).
The little knob near the bungee hooks is extraneous (was an HDTV antenna mount, a weird helical bazooka encased in plastic, sold commercially). No need to remove it.
*1 'Ellis C-2 2x4 Shore Clamps'