Turtle Shield = Dusk Mantle IMO. Turtle can't be screwed by RNG while Dusk can, so Turtle is more reliable than Dusk. It may let the first wave of attackers to pass through, but it also delays them and activated abilities can't be played. Furthermorem in a theoretical scenario of infinite turns, both shields block the same numbers of attacks (50%), but Turtle blocks abilities while Dusk blocks weapons. If Turtle delays weapons, then it would become OP. It won't only block a 50% of weapon's attacks, it will also prevent the from using their abilities for 1 turn. That hurts Owl's Eye, Lobotomizer, Trident, Pulverizer and Eternity a lot.
I personally prefer Procrastination | Turtle Shield more than Dusk Mantle | Improved Dusk due to its reliability and I don't think any buff is needed.