The only way this would work well is if you have a fuselage and covering system with no conductive items. No metal tubing, no metallic elements to the dope/paint, and no conductive composite materials surrounding the antenna. These items dampen the signal being transmitted from the antenna.
Years ago I built and tried a vertical dipole antenna mounted in the rear fuselage of a Cub. In the air testing over the regular Comant VHF whip constantly favored the exposed vertical whip with a good ground plane underneath.
Edit: We also tested the vertical dipole outside the plane on the ground against the whip on the wing. Both worked well enough to tell no difference.
Perhaps start by saying if this is required ADS-B Out installation or an optional ADS-B In installation. Then, if a required ADS-B Out installation, is it mode S or UAT? There are no performance requirements for "In" but there are for "Out".
Yup. My uAvionix Echo UAT unit is mounted to an aluminum plate under the baggage bag behind the rear seat in my Cub Clone. The antenna is mounted to the bottom side of the Aluminum mounting plate with the unit, so is about 8 inches above the belly of the plane. The GPS antenna is mounted up over passenger seat in the skylight. My fabric is ceconite with nitrate, butyrate, and butyrate mixed with silver paste, then a polyurethane color coat if that matters. It all works just fine (both in and out) and passes the PAPR tests consistently. Could the metal around it cause some attenuation? I'd just about bet that it does. But I can't tell any difference in the testing or reception between it and the other two planes I have had with Echo UATs mounted with external antenna.
I mounted it that way so I could mount up the cargo pod without having the ADS-B antenna in the way.
My ADSB-In receiver with GPS is mounted back in the battery compartment on my Super cub and works perfectly connected to my 796 via Bluetooth, sure beats having all that crap up on the dash plus no Garmin fickle finger of fate sticking up.