Looking for 1/144 scale twin 40mm mounts

Surprised you have not considered HR Products's line of fittings. Some of the castings are rough and require cleanup, some not so much, but these would seem to fit your prerequisites for stand-off scale. And...they are inexpensive. I have used them before as well in an attempt to doll up the exalted Lindberg Blue Devil abomination model kit... but I digress.  Give them a shot:

http://www.hrprod.com/144.html

have you thought of making your own? have to make a pile of quad 40mm mounts in 1/426 scale for a bunch of arizona conversions but have the drawings for them & a 1/72 scale single 40mm gun from wem to use as a template.