Wow... That's quite an ambitious project!

I'm not much of a leather worker myself, so I can offer very little in the way of advice, save this:

Unless you are a very experienced leather worker, I would suggest a much more simple style of sporran to try; anything with a built-up leather cantle will be tricky. I'd also mention that the Inverlochy is a very large sporran, so unless you are at least 6'4" and of a decent frame, I'd consider going for the Inverarnan style (which is esentially a smaller version of the Inverlochy).

Otherwise, best of luck to you and keep us posted.