"My previous RV-8 has a Garmin GPS antenna on the turtledeck under the canopy. Works well, as you would expect."
I have planned my Garmin IFR GPS antenna in the same spot (although on the other side of the canopy rail. The skin is curved there. Did you level the antenna or provide some support under the edges to account for the sloped skin or did you find it was alright as is?