Hey Mate,
I'd be happy to have a go at this for you, An existing part, with modifications is fairly easy. I'm not a 3d wiz but I can accurately draw the part in 2d, all plan and elevations, sections if required, this can then be punted off to a manufacturer or mould designer and produced, one offs tend to be expensive, there are plenty of overseas companies, mostly in China, that will do anything, if you do a run of say 20 or whatever, This is a sideline for me and I have helped out a couple of people around the world put their ideas on paper, the fee is low and you'll get people bidding peanuts, whether or not they can produce the drawings you need, I don;t know, I'm sure there are plenty of more talented operators out there than me. I am a structural drafter in my real life, so if this suits you. message me and I'll give you my email.
Cheers,
Brett