I have an idea for a bike saddle but obviously don’t have an original to make a mold from. I have a saddle that is similar but but need adapting, would it be best to add clay to this or start from scratch with some Low density model board…I am a little concerned about mirroring both side of the saddle the same.
It is either hand skills (templates, elbow grease) regardless of the method used (tooling board or adapting a suitable master), or go the computer route, which needs completely different skills.
personally id go the route with adding the clay. if you already have a base to start off, why not work of that vs starting from scratch?
You can look up how sculptors work, and what tools they use. Many types of calipers and templates, so they can reproduce a model. I would think that trying to mirror a part would be the same type of techniques.
^that’s the right idea Riff, go the sculptors route. I’d personally make a plaster replica of the seat you have that’s close, then add the clay to that. I think that with a good eye and some patience you’ll be able to mirror the sculpt just fine
When we realise a motorcycle prototype, we use only clay.
It’s the best solution and less expensive.
You just need an oven and some tools to sculpt the clay.
If you just make some modification on initial part, you can put mastic and some thin sheets of fiberglass, all depend of you work 
Hello. But clay, could change shape when it will cure? Is it really cheaper to model on foam?
I think it depends on what type of clay you use. If you use a modeling clay you should be fine. I have bought clay before and it’s hardened and cracked but there are clays that don’t harden at all and stay soft.
Interesting, besides a mold made of clay endure very high temperatures over 150 degrees easily, right? much higher than those supported by foams.
Greetings
The clay must not exceed 60°, the reason is that clay is compound of sulfur and upper this temperature, the clay cooked and release the sulfur.
When I say clay, it’s the real clay that we use in automotive design. You have different hardness, soft, medium, hard and you can apply primer, paint and top coat like an hard body.
With polyester resin, you can have some problems because the catalyse have an higher temperature than the epoxy resin.
Clay modelling is great, if you know how to work it, without the right tools
and skills you will be better of doing a hard / foam model.
… says CLAYman… 
Clay modelling is not hard, especially with the wax-like modelling clays. When working with it, I mostly use the heat from my hands, and a large kitchen knife…
We must be talking about different things here, to form something that looks ok with hobby plasticine is not hard.
But to make an original with class A surfaces using “car-clay” is something else.
Automotive clay modeller by trade.
No, it will never be a hard material, but for making an original, to be moulded off, it is plenty strong for the purpose.
And it is easy to work with.