I was able to find a used sewing machine table with a drop-in shelf for $15.00. The opening was too small for my Singer Confidence 7470, but my husband was able to enlarge the opening. He also had to notch out a place where the cords plug in. I don't use the free arm at all and so all has been well until now. I have just started free-motion quilting with this machine. The "brilliant" engineer that designed this machine put the feed-dog lever in the back and under the cover of the free arm. So, I have to lift the machine out, pull off the cover of the free arm, reach behind the machine and move the feed-dog lever, then replace the cover, put the machine back down and then quilt. After I finish, I have to do the same thing in reverse. On my 41 year old Kenmore that needs repairs, the feed-dog lever is right at the front of the machine above where it fits in the cabinet. New and improved is not always better.
As for building something removable to put around your machine on top of your table, that wouldn't work for me unless I cut the legs off the table or desk. I'm short (5'4") and sewing with a machine on top of a table isn't fun for me. In fact, it gets painful after a while.
Hope someone comes up with a solution that will work for us all.