Consider visiting a local high school, technical school, or community college with a machine shop facility, Steve. Some understanding teacher or instructor may be willing to help you to accomplish what you're 'efforting' to do. It's not a real involved undertaking, but will require some time to accomplish.
The moveable overarm on a vertical spindle mill, and the X and Y axis on the moveable milling table should make the process much easier to do than what it sounds like you are proposing.
As you may know, a vertical spindle mill looks much like a fancy drill press. The moveable overarm permits changing the throat capability of the machine. Put another way, the moveable overarm should permit you to maneuver the engine turning tool to be positioned as needed over the entire 24 inch width even with a modest sized milling machine. This capability would permit positioning and securing the aluminum piece in place on the milling table possibly just once depending upon the total travel capability of the mill table.
The graduated hand feed dials on the table, usually in thousands of an inch, will permit easily moving the aluminum piece both in-or-out, or left-or-right, for each 'swirl'.
You may need to laminate two pieces of 3/4 plywood together to assure having a rigid enough support surface for the full 24 inch width to do the engine turning process. Some pressure is exerted against the material and support surface when using the hand fed quill to accomplish each 'swirl'. Flexing of the surface not supported directly by the mill table would result in the swirl pattern varying.
Consider making the aluminum piece slightly oversize for purposes of securing it to the plywood. After doing the engine turning work, the aluminum can be trimmed to final size on a sheet metal squaring shear. Also consider making the plywood support board longer than the aluminum piece to allow for securing it to the milling machine table.
In working on the milling machine, whoever you work with will alert you to 'slop' (back-lash) in the table feed screw threads. This will be an issue if you accidentally exceed your desired feed amount and desire to 'back-up', or if you happen to turn the wheel the wrong way, and have to back-up to properly position the engine turning tool. This is not a big deal, just something to become familiar and comfortable with.
Food for thought. Hope it helps.
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York