XP rewards are very low despite how usefull shield is. I've seen couple simillar thoughts so I'm sure these suggestions will be okay.
The actual "Shield" reward is the most logical way to reward Turtle. It's fine but the only thing I would change in it is reducing the frequency of seeing low xp rewards like "Shield +1XP" that floods xp log. But I'm just nit-picking.
+30 XP for enemy killed through your shield by a teammate.
I've heard it would be difficult to do/code, is it true? Anyway, these situation are common, your teammate hides behind shield and shot at the enemy, why not to be rewarded for that?
+30 XP for enemy killed who recently shot at your shield and was killed by a teammate.