I picked up a Dewalt 740 / 7740 RAS (the arm says 740, the base 7740). It
needs a new top, but I expected that.

I'm planning on using 3/4" MDF with a melamine coating. I haven't decided
whether to try a double-layer top right off, or go with just the one and
get some experience. I'm leaning towards the simple single-layer design,
then working on a more complex design after I get a feel for the saw. The
frame is 20" wide.

How far can I extend the top without adding more support? I think 6" on
either side would be ok, but would 10" work?

Puckdropper
--
Make it to fit, don't make it fit.