I went with the Acme thread approach. I needed adjustable jack stands for my 1967 Mustang Fastback project and something that would support stock for my bandsaw. I bought four of these. They originally came with rollers on them. I threw the original roller attachments away and made new attachments that were more versatile. I've since made two attachments for them. One is the roller so I can use it to support stock for my bandsaw. The second is an an attachment I use when I'm using them as jack stands.
Here it is in the configuration to support stock for my bandsaw.
Here it is in the configuration for use as a jack stand.
Here are all the parts that make it work. The Acme thread slips into a tube that is slipped into the stand. The tube is held in place by friction with the large washer and a large set screw (not shown in this picture but visible above). At it's lowest level the bottom of the slip tube rests on the stand. The Acme thread is adjusted with the Acme nut. I can then lock the Acme thread in place with the set screw as long as the slip tube is at it's lowest point (note hole in slip tube). The different "attachments" attach by a single flat-head 3/8-16 bolt.
I have two more that are taller that I use to support stock for my drill press. I also use the taller ones as an "out feed table" for my table saw.
In the future I may make a "V-type attachment to support round stock. When other needs come up I can fabricate an attachment to accommodate them.
Britt