You could also do it with aliginate to make the mould, cures quickly (+/- 10 minutes) and costs around 10€/kg dentist use it to mould the shape of your mouth.the problem might be, It can only be used once. the advantage is you can have some undercut without problems and there wont be a seam and you"ll see every detail
You mix your alginate, put you hand/arm in it, let it cure, remove your hand/arm, cast with polyurethane, remove the alginate by tearing it apart and you have a detailed replica of you arm
check some videos on youtube!