I found a skeleton spawner that was 26 blocks from the surface. As they need to fall 23 blocks for a mob trap it meant I could incorporate it with some sort of surface structure. To keep this in theme I've just finished building a massive Skull Hall sandstone skull wearing a stone helmet.
The helmet has sandstone horns. I have slightly messed up the tips of the horns so will need to try and figure out what I did differently on each side. This is where flying would have been an asset.
Below the skull is the skeleton mob trap where you can quickly gain XP (around a level a minute I guess) and can also grab heaps of loot. Please feel free to make use of this.
You can reach this by taking the spawn portal. Once you reach my portal head North along the short bridge. You will see the Skull Hall in front of you.
Also, I don't know if others are struggling with sand and sandstone but there is a desert just a short distance from the hall to the North West.