From your photo, your frame has a disc brake tab on it so you don’t need a d’brake, just the proper size adapter for your brake – you don’t need to make anything, just get the correct adapter and bolt it on. The NightFox didn’t originally have a tab so you needed a d’brake with those (as per the photo in your original post).
You probably need an IS to Post Mount adapter depending on the brake you have and size of the disc, I can’t remember at the moment if the mount on the frame is IS-Front or IS-Rear (I think it is IS-Front but you’d need to check). If in doubt get a bike shop to sort you out, these are standard bike parts and not particularly expensive.
With respect to the panniers, check with UDC if you can insert the rear part of a Nimbus Shadow Handle in your frame. That would allow you to hang stuff off the back pretty securely. The front handle is, as far as I know, the front of a Shadow Handle set, and the seat mount is very similar, it is just that it is part of the frame rather than a pivotal seat mount like the ‘conventional’ Shadow Handle one. That said, I’ve never seen a NightFox or Shadow Handle ‘in the flesh’.
Edit: Here is some stuff I said about disc brake adapters in the past: