Hi Klaus, I don't see any reason that the unit can't be built as you have shown. The pyramid shaped casting is attached to the enlarger, and the box shaped riser lifts off in my enlarger, but I can't see why they can't be made as one piece. One reason the top piece of the original may be square for a few inches below the light source is that there is a tray and access slot in the original box, presumably for filters, I don't really know, I've never used it for anything. The slope of your model would keep full size 12"x12" filters from being placed under the light source like I think the original part allows.