I can guarantee the original caps aren’t airtight either. That’s not what they’re designed for. The valve is designed to be airtight. The cap only stops dust building up.
A good cap will keep a decent amount of air in and protect the valve from damage. I wouldn’t trust a 3d printed cap that I don’t know who made and how good their QC is. If I made it myself though I’d be less hesitant. Either way, it’s better to 3d print covers that go over the originals.
I can guarantee the original caps aren’t airtight either. That’s not what they’re designed for. The valve is designed to be airtight. The cap only stops dust building up.
A good cap will keep a decent amount of air in and protect the valve from damage. I wouldn’t trust a 3d printed cap that I don’t know who made and how good their QC is. If I made it myself though I’d be less hesitant. Either way, it’s better to 3d print covers that go over the originals.