You are right. The small part is the corner under the hood and the big mould is the outside fender. I asked but I am not allowed to post pictures of the project (I have only pictures from one fender)

Ah okay that makes sense. This would be for like where the fender would bolt to the car under the hood? And the large part of the mold would be the outside of the fender that you see? You wouldn't happen to have any in process photos of your own would you?

Attached a picture what I mean. You put the loose part on the flange so that it is inside the bag which is sealed on the flange of the big mould. Important is not to drill holes through both parts. put a threaded insert in the lower mould so that it is airtight when you fix the split part on the big mould.

Also interested to see how you put the split on the mould flange? 

Yeah, I am going to get started on something a little easier than this fender. I am wanting to start doing infusion by making a door skin. It is relatively flat except where the door handle goes. 

Hello,

we and customers have made several fenders. It is not so complex. The heaviest thing is to make a split mould. I prefer to put the split part on a mould flange so that it is enclosed the bag. Tham minimizes the possible spots for leaks.
If you work accurate you will get perfect results with corners like the original. Attached a link to pictures from a customer who made fenders with infusion and MTI. The parts are just trimmed and polished.
Audi S3 fenders

to be honest this looks like a very hard/complex to start testing infusion or any technique with  

can you explain what you mean by have the returns? Where were you thinking of breaking the mold up? Where both flanges meet the outer part of the fender?

A multi part mould will allow you to have the returns, although you will need some sort of spray tack to hold the fabric in place as you work

Here is a picture of a fender off a b5 audi rs4. I am having some shipping from Germany to the US. I would like to make a mold and then use the vacuum infusion method to make some CF ones for my car. I have not done Vacuum bagging and I know this is probably complex. I am working on easier smaller projects to learn, but I like to have time to think ahead about this. In the lower left corner you can see the flange that bolts under the hood, and on the top you can see the flange that bolts by the door. My question is how do you get the insides of those to have nice sharp edges like factory. Will the vacuum have enough pull to get the fabric and resin into those spots? I have searched all over here and the internet in general for anyone that has made fenders and have come up dry. I know these exact ones can be done as a company sells carbon fibers ones for a ton of money.