I think that everyone who has messed much with treadles has dreamed of the idea of a top you could change to put different machines in. It isn't that hard to do. I originally built a Rube Goldberg system, with a slotted metal back bar, into which the hinges went with bolts and butterfly nuts underneath, so that any lateral pin placement could be accommodated. This bar also slid forward and backwards enough to close the gap, which was of course cut oversize, so that the front edge of the machine could engage the lip. It worked, but inevitably there were some gaps around the ends of the machine head.
My Mark II concept is actually much better and easier, though I haven't built it yet. It would simply be to make an oversize gap in the table top, probably using the 3/4" craft plywood to obtain a nice large surface. I would then mark out a rectangle about an inch larger all around than this opening, and line that marked edge with a 3/4" deep molding. Then I would cut a piece of the plywood to a rectangle that exactly fit into the molding. If you haven't guessed the next step, you ought not be attempting this project! Just cut your sewing machine opening into the removable rectangle of plywood, aligning the belt and everything properly. Cut half a dozen spare rectangles, and whenever you get an odd machine you want to try, mount it in it's own rectangle. All you have to do to change machines is pull one rectangle out and put in another. It wouldn't take too long to end up with just a few interchangeable bases that covered the needs of most machines… Singer, White, Davis, National, Standard and New Home would be the main ones. Most odd brand names end up being badge machines made by one or another of these companies anyway.
This fine example of a universal top was submitted by Shirley Thobe and Ken Robinson. The Necchi is a BDA from the 40s. The base is a no-name iron with wood pitman. The top is birch plywood stained with a mahogony stain that gave a reddish color. Ken made the inset to fit the Necchi, then future insets will fit around that particular head and line it up with the flywheel. The gold foil decals were made on the computer and printed on a specialized printer, fit into the corners of the top, and decorated with the S (for Shirley not Singer).