Sand, anvil, etc will turn into an entitiy when placed on top of shulkers, and can be stacked up to 2 blocks. Doesn't matter if the shulker positioning was sideways or straight. You can place the block by making a platform above the shulkers, to make it falls directly towards them.

If you keep placing the block for a while, it will get broken, it may fall into the shulker, or to the side of shulker.