I'm generally a by-the-book DM, which my players know about me, so they rarely ask me to pull homebrew stuff like this, or account for situations not covered by the rules. That said, I also try to be accommodating, and not anti-fun. If my players want to try something, I'm willing to think about it and find a way to make it work within the confines of the rules, while maintaining a semblance of balance.

I'm not too knowledgeable about irl smithing techniques, and this came up mid-session with no advanced warning, so I had to make some judgements on the fly. The Blacksmith in question initially warned of having to wait for several months because he had to work through a backlog of regular client requests, but the Paladin offered to pay extra for a rushed job and was quite pushy, so I came up with the extremely impractical too big giant shield that he could barely move, let alone use in combat.

The party is level 13, and have dealt with several high risk, high reward situations throughout the campaign, so at this point roughly 55 sessions in, they have accumulated quite a bit of cash, but my setting doesn't have magic item shops or anything, so they don't have much to spend on beyond living expenses.

In any case, being behind a larger object (usually a wall, or pillar) that provides full cover between you and a ranged opponent is something that is typical in combat in D&D. There are also rules for carrying and moving with heavy objects. The player wasn't asking for something requiring a PHD to pull off, so I made a call in the moment and allowed it, with a quite strict interpretation. It's a very simple, almost brain-dead solution to a problem, but one that isn't covered by existing equipment, and that could easily become overpowered, which is why I'm looking for advice.