a trick to avoid hammering is to go around a bit at a time and 'squeeze' the rubber top/bottom so the slit that goes over the sheet metal opens up - also bend the rubber back toward you as you fit it on, which also tends to open that slot
the aftermarket headliners are plastic, where the originals are a fabric with jute-like backing that may not have gone quite to the edge, so yes, likely the repros are thicker, I've never used one, always saved good originals
Bill
