What tool should I use to cut the
Himax door out of the fuselage?

wouldnt something like a multi tool (oscillating tool) with a fine blade make the smallest cut?

I'm personally a big fan of "Japanese pull saws" . Their teeth are backwards so they cut on the pull not on the push stroke and that lets them have very thin blades that are very controllable.   For precision cuts I have a couple of scrap sticks that I've run through a thickness planner to make them perfectly square, I clamp one along the line I want to cut then hold the blade against it with one hand while pulling it with the other, it makes perfect square cuts with a kerf narrower than a standard pencil line (about the width of mechanical pencil line).

I concur.  I do all cutting with pull saws.  They do beautiful work and they cut very fast because the blade is so thin.

Tom

Thanks for the response guys.

I haven't seen the highmax door details (I unfortunately didn't down load the plans before team started charging), but assume that the side is built as a unit then the door is cut free, if going with a very precise narrow curf cut you may have to slightly angle the cut to allow the door to open without binding.  A few degrees of angle making the outside ever so slightly larger than the inside to let it both swing out and seat nicely when closed with a very thin (felt?) weatherstripping installed.

A flush cut saw might be the weapon of choice. It is very thin and cuts on the draw stroke and has no set in the teeth so the curf is no wider than the blade. They are inexpensive and most all hardware stores will have one.