Homebrew PCBs

I've done kapton, brass sheets, and aluminum foil.

The kapton's nice if you need accuracy, but I think it'll be important
to ask for vector-cut stencils instead of raster-cut for fine pitch
parts.  That stencil lasted through eight boards, you just have to
carefully cleen it with something that cleans your solder paste.

brass sheets are hard to etch because of the thickness, even a 2 mil
sheet gets 2-3 mil undercutting which makes it very difficult to get
accurate stencils for fine pitch parts, or even 0603s.

The aluminum foil method[*] works great if you can align the two masks
properly.  A 1 mil foil gives a 4 mil stencil, but it's rather flimsy
and really only good for one, maybe two boards.

As for 8 mil alumimum, I think it would work but you'd be limited to
larger parts.  Since you're milling and can get straight sides, you'll
have to experiment and see how small a hole you can mill and still
have the paste stay on the pcb when you lift the stencil.  Also, you
can make the hole *smaller* than the pad to control the *volume* of
paste put on the board.

[*] Put photomask on both sides of household aluminum foil, expose and
develop.  Etches the foil in a few seconds, leave the photofilm ON.