Hi Gonch,
Have you had a look at the 'Fixed Installation' Section of the Forum? Although there is only an enquiry so far about a PA28 fixed installation from 2018, there is a lot of information from other users which you will find relevant.
For my part, I strongly recommend that you investigate fitting external antennas on the underside of your aircraft if you are able to do so.
Having tested many other options over the years (since 2015), I now get much better coverage using external antennas (even in my flexwing) fitted under the pod with a suitable internal ground plane, than I have had with any previous antenna option or combination. I have attached a diagram of my P3i coverage for comparison.
Be aware that the apparent bias to the left in my installation is 'probably' due to the proximity of a second identical antenna used for my PowerFlarm transceiver - mounted underneath the right hand side of the pod (the P3i one is mounted under the left side). Unfortunately there is limited space on the underside of a flexwing and I already have a transponder antenna to the rear.
My advice is always to try to separate antennas as far as practicable from each other - especially if there is a potential frequency conflict, such as between the transponder and your PAW 1090 Rx antenna and take care to avoid obvious 'blocking' from undercarriage legs etc. by siting the antennas away from such obstructions where thay will have a good view of the ground and if anything biased towards the front of the aircraft.
If you are unable to fit external antennas, reasonable results can be obtained by using internal antennas mounted at the sides of the windscreen or in the side windows, clear of the door / window frames.
Let us know how you get on.
Best Regards
Peter
