A few builders have used hidden springs to hold the fairings into place. Very slick but I'm not that clever.
I'm sure there are any number of way of approaching the problem. At the time, I could not find much info on the subject either so this is what I ended up doing. Fairings-Etc. recommended the super simple technique of merely using self tapping sheet metal screws. In this case, I took advantage of the removable access plate and installed one nutplate. That ties into a machine screw through the forward end of the intersection fairing. On the aft end of the fairing, I used either 1 or 2 (I cannot immediately recall) self-tapping sheet metal screws through the fairing and into the fuselage bottom. Because the attach screws are located underneath the fuselage, the fastening method is not readily apparent to the casual viewer.