ProDave I have strengthened my ceiling pvc panels where the downlights are fitted with a 200x200x5mm perspex patch with a 75mm dia hole siliconed to the back of the panel. (I have a CO2 laser cutter so this was an easy task) This could also be acheived with plywood etc. If you can reach the top side of the pvc panel you could cut the patch in half and put the the two pieces together with the light in situ.