great suggestions- the 5200 is painted- bottom paint- since its below water, not much sun. I agree, i could have/should fiberglassed them- i wasn't so concerned about making it water tight for the first mount, my concern was mainly ( from transom mounted transducer experience- when the fail, they are a paint remove, replace) what would happen if lights failed and had to be replaced- remove lights; break 5200 seal, now holes are larger,The hole for the light wires would have also had 5200 as it would pass through the transom below the water line- it would have to be drilled out. So i figured beating up the blocks if the lights needed to be changed or if better lights came on the market, the blocks up would be a better alternative. I am not totally paranoid about making holes in the boat ( I already have 5 seacocks and two thru hull transducers) As a side benefit, I was also able to build an angle into the blocks to better direct the lights across the back of the boat and cover more area.
As for removal, I have found that 5200 that is around screws it is pretty easy to break the bond if you use a heat gun before you attack the screws.
But I do agree, fiberglassing the blocks take it one step better, if you truly what to be watertight.