Are they mounted in a trim or gasket? Or are they flush mounted and bedded into the cabin top in rabbits?
Mine were flush mounted and had to be removed by cutting the bond on the inside first with a 2" putty knife. It was an older installation and the bedding had gone hard. When the lens came out, the starboard was in two pieces and the port in three. It also made a mess of the outside of the cabin top when it came out.
The pieces were dropped off at Select Plastics in Norwalk Conn (Shameless plug for the guys who have done all the lenses in my Lewmar Ocean hatches) and they had new ones cut in a week out of smoked Acrylic.
They were re-bedded with Dow Corning 795 black construction adhesive. This is a slow set bedding and needed to be clamped for a week. The outside was re-faired with fiberglass filler and painted. Labor intensive to say the least, but mine came out OK.