Here is a background damage block similar to the one used in my summer contest hack. It's currently set to do 1 HP damage per second for every damage block Samus is in contact with (so a max of 3 damage per second if she's standing in a load of them).
It uses BTS value $01 for air fool x-ray (so you can use end game BTS and stuff as well, yay) just don't place them near grapple blocks as grappling while in contact with them causes crazy damage.