Cut little squares of the black and white Mylar and place them on the top of the blocks, works a treat. It will keep them in perfect condition and you can re use them over and over. Its a little trick I have used for years
great advice from above, i do the same thing with my coco, ive cut small plastic circles with slits that fit lovely round my stem and cover my coco, help stops fungi growth and keeps them fungi gnats away,