I've seen online where people have bought thick foam to surround their machine and bring the surface up to flush, usually with a layer of vinyl sort of "tented" over the top to hold everything in place and provide some slip. (Hole cut out for needle, of course)
If you can find a desk with a deep enough drawer in the right spot, it might work to cut a hole in the desktop and let your machine sit in the drawer; just shim it up to be level with the top.
What if you took some plywood and cut it into a shape that would fit around your machine, you could use wooden dowels or similar to make legs for it, and it could just sit on top of whatever surface you're sewing on? Like a giant extension bed. You could laminate the top with contact paper for better slip. Or a good coat of polyurethane might also work?