Cathy, your cabinet and inserts are great! Thank you so much for sharing. You certainly have "raised the bar" compared what my plans were - LOL. I'll have a setup kind of like yours in that I'll have a treadle base top with supports built in. Then each machine will have a base to fit into the universal top. I don't think I'll have any hinges though. I'll have to put my machines on a table top to oil and service them. Your idea is much better.
I too was thinking about how the treadle belts will need to be different sizes. I think I'll just take the easy route and have a belt for each head. I have lots of oxygen tubes so I may as well use them up. Again, your plan to have an adjuster is an elegant idea.
I haven't started making my base yet. I had a quilt I needed to get in the mail and now another need for a quilt has arisen. Duty before fun! Again, thanks for sharing Cathy. It was fun to see your pictures and hear about your universal treadle and the machines you'll put in it.
Judy