Buy 150 forts to begin. They can be low quality. I believe it is MRMB that then gives the biggest boost. Ammo doesn't boost your DI but you will need the ammo that goes with whatever units you buy.
I have a company that makes 1 per month and allocated the full production to the country. The game says that the contract wasn't kept because the contract exceeds the production. I had reserved 1 from the production--same message. I zero out the reserve and I get the same message.
The GM may have simply disabled delivery of contracted forts to countries. In any case, there is no difference between a 120 quality fort and one of higher quality. There is no reason to contract yourself forts over simply purchasing them from the market at 120 quality.