I'm sorry to have missed a shield discussion -- it's my favorite topic.
Don't forget that shields are to be used in melee as well as to block arrows...so you have to balance the coverage with the durability and speed of the shield.
Having extra shield skill increases the "forcefield" of your shield. So with 4 or 5 shield skill, even the narrower shields will provide plenty of blockage against arrows. Because of this, there is no need for most round shields. Why? Round shields are wider, but because of that they sacrifice durability and/or speed to keep them balanced.
Elite cav shield is made of paper. If you want your shield to break in three hits then maybe you should use it. The "knighly" shields that many prefer are slightly better, but still weak. Heavy round shield is only mediocre -- lacking in speed and durability for what it is.
The worthwhile best-balanced shields are:
Kite shield (for low shield skill)
Heavy Kite Shield
Heavy Heater Shield
They are all tough shields, with plenty of speed. Best all-arounders that will save your leathery, scaly hide.
Next up, consider the toughest shields. I know you've tried some slower shields -- but keep in mind you have to adjust your playstyle a little when using them. You must practice patience and not fall for feints -- slow and steady, young terrapin.
Heavy Board Shield
Plate covered Round Shield
Steel shield
+other weird new ones
The metal shields have very high armor. Unless your opponent has an axe, you can safely let them beat on your shield for as long as it takes to find an opening to attack. Frustrating the enemy with a nigh-invincible shield usually makes them over-extend themselves and mess up. The Heavy Board shield (and those with similar stats like Pavise and Triangle shield) have very high hitpoints and fare better against axes.
BONUS: Shield weight slows you down a bit, but helps resist crush-through weapons. Being able to potentially block mauls (the bane of all turtles) is very nice.