OK Guys, The Bench Top Mill, here goes ... The plans call for a sub base of Aluminium Extrusion (ITEM) 28mm x 240mm x 500mm. As a temporary stop gap part, how about a 25mm x 240mm x 500mm thick MDF slab sheathed with a 4mm aluminium plate above and below. The hight is not critical as it serves as a base only, with two rows of 8mm through holes spaced 50 mm apart and 22.5 mm in from each long side.

If it would help I could bond the sheet and MDF with epoxy and squash it between my table and a further pair pair of old surface plates.