if you are at all handy you could drill and tap a hole into the top of the joysticks, and install a bolt with rubber tubing over the threads, to grip, in a length that suits you. Would take about 10 minutes...If you do not have a tap and die set just take the bolt and use a Dremel cutting blade to score a progressively deeper cut 1/3 way or up the shaft or up the threaded shaft. It's just plastic...I looked at my Mavic's removable joysticks, not recommended.
FYI I tried the CarolinaDronz.com site to order the knob caps, they refunded my order and said they're out of stock. I found almost exactly the same knobs on Amazon for 3x as much but at $10 it still isn't bad. They also showed many comparable products if you don't like this option.