The other options are to try casting,
thermoforming, or fiberglass modeling, or stamping
some of the parts.
If you can cast them from liquid rubber/plastic, just make
a mold from clay or such and pour away then trim when
cured.
If you thermoform, just make a mold out of something like
clay, plaster, sand & glue, or whatever, take the
appropriate semi-thin sheet of thermoformable plastic,
drape it over the mold, heat gently (e.g. hair dryer,
heat gun, or very carefully controlled time in the oven on
low heat), and apply gentle pressure to form the pliable
soft warm/hot plastic over the shape you're trying to
mold it to fit. Cool and trim.
Fiberglass modeling is like paper mache, just get your
resin and some small pieces of matrix (fiber glass cloth,
cheese cloth, gauze, newspaper, whatever is appropriate),
dip/paint them with a thin coat of the resin, let dry.
Stamping, just like using cookie cutters to make
shaped cookies. Make the outline of a stamp shaped
like the part you want in something like sharp sheet
metal from a tin can, or carve the outline with a dremel
tool, sharpen the edges, and press it hard against a
relatively soft / thin plastic sheet to make as many
copies of that cutout shape as you need. Works well
with stuff like soft vinyl, thin HDPE (plastic milk jug), styrofoam, rubber, teflon, etc.
Some kinds of plastic like delrin, teflon, HDPE,
paraffin wax, etc. you can just directly carve and file
and cut with tools like x-acto knife, dremel / rotary tool,
files, etc.
