Lots of great ideas here. Here are a couple things I can add:
1. While I typically buy authentic Manfrotto products, I purchased this knock-off of the Dado simply because it is black. The only possible deficiency is that the finish may not be as durable long-term. This is a non-issue for me as I use 7/8" neoprene washers on all of my male 3/8" connections, which have the added benefit of keeping everything very tight.
Magic Ball - Ali Express
Magic Ball - eBay
2. I have recently become a big fan of
9.Solutions products. 9.Solutions makes some very nice
3/8" and 5/8" rods in various lengths from 150 mm to 1 m. The 3/8" rods are slightly larger than 3/8" diameter, but have 3/8" male threads on each end. The 5/8" rods have 3/8" female threads on each end, and use some very clever double-ended rod connectors for attachment. The rods also have a flat indent on each end for interfacing with a 5/8" pin socket.
I believe 15 mm clamps such as
those mentioned earlier should still fit on a 5/8" rod, as they are less than 1 mm wider.
Tangential to this thread: If you need any kind of rigging clamps, you should give 9.Solutions serious consideration.
They have a wide variety of options. The
Savior Clamp With Socket is the greatest clamp since sliced bread, in my opinion.