Urethane will do OK. What you really want is castable/pourable silicone. Usually die making supply companies sell them in cans. The industrial supply company, W. W. Grainger sells a two part urethane thatworks decnt, but you need to also buy the additive that reduces the durometer, or they will come out about 80 duro which is too hard to keep from cracking.
I know I sound like a broken record....but I actually have been working on the castings. Until I can move all of my stuff out of storage next week, its difficult to do more than make the basic mold supports.
Here is what I am working toward. There will be several basic sets.
I am setting them up to mold in sets, so as not to have piles of dies layng around. Its easier to mold several parts at once under vacuum pressure and them trim away the "sprues" and feeders...carefully.
Set one: This is a front end suspension set. The silicone parts made in one pull are: Boots to cover tied rod ends and center links and new strut boots to go with new beetle bump stops, on rebuilt struts.
Set two: The urethane parts: That would be control arm bushings, subframe donuts, radius arm donuts, front and end sway bar links.
Set three: Front body: These would include...bellows with o-rings for wires to parking lights through fenders, gas tank boot with optional gas pump handle pad, seal cords for around headlight bezels, antenna bung for anntena cord through body, door handle gaskets,.....and...both early and late bumper stanchion rubber covers....

something sorely needed but never offered on the cars.
Set 4: There may be two molds. These are rears. Mostly engine. Include oil pressure sending unit gasket, the auxiliary air reg take off tee, manual shifter bellows, rubber cuffs for the chimneys to the heat exchangers, heater cable covers, rear tail light gaskets, license plate light gaskets, bellows for tail light wires, oil filler bellows for wagons.
And yes....a seperate mold for bellows for both fast and squareback for the air intake. Also, two different models of TB to aircleaner tubes. I will also make a seperate molding on the same box, to make intake tubes for to use a 1.7L boot on a car that has a 2.0 TB.
Other longer term projects include....acrylic fender welting by the roll...both styles. Rear trailing arm bushings of delrin with offset bores in 3 dimesnions to correct rear camber, door trim...copies of the original chrome...but cast of textured black silicone...with a stifner strip cast in...and held on by .040" 3M 486 adhesive.
The copper plate mold is done, just waiting to get the aluminum tube cut to make the crimp rings.
There are others too.......including....cast, hard bodied....felt lined, molded front end bras for 411/412.....ooooh baby!
If all goes well.....I should be molding in ernest in the evenings by may.
Yes.....I needt to paint and send out your centerlink this wekend. Sorry...my shop premises are just a little in limbo at the moment. Ray