Bastion spawner magma cream farm

Magma cream from the magma cube spawner found in some bastions. You clear a room around the spawner so the cubes can spawn, an iron golem and a few wither roses kill them, and hopper minecarts on rails pick up the drops. Spawners ignore the mob cap, so it suits a multiplayer server. Kill the cubes yourself to get XP as well. Not every bastion has a magma cube spawner, so finding one comes first.

• Clear 6 blocks around the spawner and 11 below it. For the space above, a comment with 96 likes says 5 blocks, one with 44 says 3: check the video at 4:00 and 7:15.
• No need to fill the floor with wither roses: the cubes walk to the iron golem, so rails under a small area catch the drops. Several replies say so.