I guess when the took MacGyver off the air people lost all their creativity. With a leatherman and a roll of duct tape you can build damn near anything.
This is what I use... about $1 at the hardware store. My T-bar happened to have the 3/8" thread on the lower knobs, so I bought a 3/8" coupler nut, which is about 1.5" long. Then I wrapped some tape around a standard 3/8 bolt and wound it in there and cut it off... the tape basically fills up the threads so it binds and doesn't come off. Lock-tite would be the right way to do it if I had some on hand. I've been using this thing shown in the picture for hundreds of shows. If your piece doesn't have 3/8" thread, you could still do the same thing, you just need to add a 5/8 > 3/8 adapter on the bottom and a 3/8 to 5/8" adapter on top. Those are available cheap.
Other creative solutions I have used on occasion.
- if your knob can be unscrewed from the T-bar, unscrew one, and screw it back in from the other side, so you 1 mic above the t-bar and one below the t-bar.
- if you have a spare T-bar, or make one from a piece of flat bar, just bend a big zigzig in it with a vice.
- cut off a piece of dowel or broom handle, drill a hole down the middle... there's your spacer, go get a longer bolt...