For a back shield, you want high armor (since projectiles that hit the shield on your back see your body armor+shield's armor+10) and low weight. Thus, the Leather Covered Round Shield is the ideal back shield. When loomed, it gives you +42 armor against any projectiles hitting your back, so even loomed bodkins fired from loomed longbows will do <10 dmg (assuming you wear loomed medium armor). It's also large enough that it covers your back/head well.
The Hide Covered Round Shield is also a good if you don't have any shield skill, it offers almost as much armor for even less weight.
