Yeah, some more details and pics would help.
But heres how I would do it. Find a piece of cardboard tube that your light housing fits into. Buy a hole saw that size. Figure out where you want it to go. Drill your big hole. Cut a piece of your cardboard tube the appropriate length. Prepare the inside surface of the seat pan with some 80 grit. That will give the new resin some bite. Use a few drops of super glue and tack the tube to the inside of your seat hump. Cut a round piece of card board and cap off the back side of the tube. Go to your local AdvanceZoneBoys and buy a fiber glass repair kit. Follow the instructions and apply fiberglass mat all over the cardboard,and blend it onto your seat base, two layers at least. let it dry, clean up. drill holes for your wires to exit the new cup you've created and holes to mount the light.
Hopefully, the bezel on the light will cover the seam on the outside.