The official Theta piece seemed excessively pricey and my local hardware store had some imperial screws and nuts so I just cut them down to be precise 3 threads deep to go inside the Theta and make it flush with rim of pole. I used pipe scorer cutter for perfectly flat cut poles and glued dowels inside to secure the nuts. The tiny leash is looped around fingers to give more security.
When holding it in a microphone position when filming it all disappears in the shot so no post production needed to remove the nadir.
Ive got a similar setup for a face height monopole on an extendable music stand which just stacks by gravity on tubes half inside each other but still is airplane carry on luggage portable.
My favourite hack for a tripod is still the suspended inverted theta on a fishing line to a stud 1/4 flap screw in the base.