I'd be wary of the shear pin idea to be honest, you could design it for normal steering loads but then it might fail under additional load such as kerbing or large pot holes.

If you were to design it with these loads in mind then you run the risk of it not shearing in an accident.

It's feasable to design your own steering column but given the inconvenience and possible SVA pitfalls I think you're better using a production car item.

cheers

Chris