How do I make a valid respawn anchor

I tried using the respawn anchor In the nether and it seems to only work at random. I put warped nylium on 2 of the sides and air on the others but it says the anchor is still obstructed and when I respawn in the overworld after the anchor fails I cant press buttons or move for a bit. How do I make an %100 guarantee that the respawn will work? Or in other words how do I make a valid respawn point with the respawn anchor? Please attach images for reference. Thanks. This is for MCPE or Bedrock edition single player.