I managed to source a fresnel lens as part of a ship's anchor lamp. When this arrived, I made a new cap to go over the top, as the previous one was not terribly attractive.
The larger top disk is cut from MDF using a router - much like making a dalek neck ring. The smaller dome is from a battery operated closet light and is held in place by the paint (!). I reused the black-painted brass rod from the old cap.
I also found some nice black drawer pulls, which I think look rather better than the silvery things I had before. I also picked up a brass night-latch.
I sprayed flat black paint over the handles and then grey primer paint over the top of that to give the handles a 'used' look. To weather the brass night-latch, I first cleaned it with alcohol, then rubbed it down with steel wool (actually I put the steel wool on the bench and rubbed the lock onto it to stop fragments getting into the lock cylinder), then sprayed it with the same paint solution I have for the windows (with the key in). Thanks to Banzai and TimeGirl (from The TARDIS Rebuilders) for advice on how to do this.