A shield type item that you had to slot into your spare hand for it to work could be interesting for sure, only giving a buff from that position in inventory and deteriorating from there too.
For one it might be a good idea to have more items than motion sensors as a secondary slot item, to keep variety there, and for two it could also provide debuffs, like reducing your max chance to hit by 25% or something, so that it doesn't become a "must always have on all players at all times" type item.
As for players taking 10 of them, that's fine, you just need to balance how it is utilised, say if equipping it put you at -16 AP, so that fast (re)equipping during combat was just not viable, problem solved.
Cool idea in general that wouldn't necessarily ONLY apply to a riot shield too, could have it as a weird fallout tech item, think stealthboy item that is instead a plasma shield or whatever, and also think of other items, not just shields that could be utilised like this, as a secondary slot item.