No no no. You can only make non-epic magic shields with a permanent +5 enhancement bonus. We're not creating a magic shield here. The shield doesn't have to be masterwork for you to use Divine Shield on it, and it doesn't become masterwork when you use it. It doesn't become a magic item when you use the feat, just like you don't become a magic item when you get Cat's Grace cast on you.
They're two completely different birds, albeit with similar plumage.
And if you want another argument - there's no +5 restriction on even magic shields, since +6 and higher are available in the epic handbook. You can create a magic shield with a +6 enhancement bonus with craft epic armor. Since we're not crafting a magic item, we don't need the feat, there's no caster level restriction, so it just works.
If you would like to rule as a DM that since Divine Shield is not an epic feat, that it can't duplicate epic effects, that's your decision, and perhaps not an unwise one. However, there's nothing in the epic handbook that says "no effects in the PHB or class supplements can reproduce the effects of epic magic items".
-The Souljourner