I think it should be possible to use a piece of RHS or RSJ as a starting point to make your own. It should be possible to add a couple of stiffening ribs held in place with screws if needed and then you can skim the working faces with the milling head. Add an aluminium fixture plate and you are done. If you use a plain piece of aluminium you can put in threaded holes for clamping down the vice and add more as needed for other parts. Extra plates for other uses are going to be relatively cheap. If you add a few dowels to the base you can make the plates interchangeable.
There are also a number of used cast iron cubes to be found if you look at ebay with sizes like 6" x 7" x 9" which may be a good option as well.
Martin C
Edited By Martin Connelly on 22/09/2023 12:59:48