I have also tried the use of a strip on one side, that did not work at all (in my case the thickness of the hull halves differed).
What worked for me:
I cut the strip in parts and alternating clued them to the upper and lower hulls.
I made the index lips out of polyester resin and 2 layers of 300gr fibreglass cloth. I took over Davit Merrimen’s idea to make them the way I did. First I attached some wax paper to the hull i.w.o. the indicated cut lines. On top I laid-up the fibreglass so the hull shape is exactly transited to the fibreglass. After curing the wax paper was removed and after trimming the fiberglass, you remain with very nice and easy to install "hull shaped" index lips.
My model (not yet completed) is only 30” but it might work also for you.