Hi John,
I made my own trays for use underneath my lathe & milling machine. They are made of 1mm sheet steel for the base, and 25 x 3 x 3 angle for the edges. I used pop rivets to fasten the steel sheet to the inside of the angle iron having first partially drilled a wider hole from the outside of the angle so that the pop rivet (inserted from the inside) could expand into the widened hole and the excess filed off flush. I also used a strip of bent 1mm x 20mm x 80ish mm on the inside of the corners to hold the vertical sections of the angle iron together. This strip was also fastened with pop rivets. Hardly beautiful, but very functional. The whole assembly was then painted with a very smelly green two coat type paint (avoids giving tradenames, does that!).
In the case of the lathe, this meant that the steel sheet base was 3mm above the bench top, but as I wanted a large (75ish mm) diameter washer on top of the wooden bench anyway, it was no big deal.
In respect of the milling machine, the size was dictated by the top of the trolley I was mounting it on: it had to be a minimum of 25mm wider on each edge so that the sheet steel base sat flat on the sub-base.
In both instances the tray is clamped in place by the fixing bolts for the machine concerned.
The milling machine tray is 780 x 520 mm with the front 100 mm in front of the base of the milling machine, a Warco MiniMill.
Off machine, both trays are a bit wobbly – as you might expect. But once clamped down, become very rigid indeed.
Hope that helps,
Peter G. Shaw