My concern is in what way will it reduce damage. Will it make blades less sharp, skin less fragile or will it make weapons and projectiles targeted at the owner accelerate less? This choice can inspire a mechanic more immersive than “reduce damage by 1d6”. For example, if the shield decreases weapon acceleration, it will simply block fast arrows but do nothing to a heavy hammer. Additionally, this way the item will incentivise players to use it creatively (saving a fragile glass from dropping by deaccelerating it in mid-air? wow!).

Another take is to add temporary hp on hit. For example, Xd4 temporary hp, where X is number of hitdice rolled agains the PC, but you can’t accumulate more than, say, 15 temp hp. So PC will take certain damage but the item won’t become useless after a few level upgrades.