My dm has a custom item, a buckler you can strap to your arm, it gives +1 ac and frees up your left hand, it is a shield so I want to put enhanced defense, because I had to create the item as a magic item, seeing no other way to make custom equipment I cannot add the infusion, since infusions cannot go on non magic items.

A simple workaround if you don't wish to implement custom equipment, would be to allow me to override item requirements on infusions.

Simplest option I can think of is to just use a normal Shield that you can infuse normally and then make a homebrew magic wondrous item that offsets your AC by -1 to compensate.
Modifier Type: Bonus
Modifier Subtype: Armor Class
Fixed Value: -1

Technically an even simpler option that you could achieve the same result with is by just infusing a normal shield and customising your AC by clicking on the AC shield on your character sheet - but that would never change and you'd have to track it manually. The homebrew item with -1 AC means you wouldn't have to worry about that.
You would just have to remember to remove the homebrew item if you ever got rid of the buckler - so name it appropriately.

Also - I tend to make my not-item homebrew items that simulate temporary effects or customisations Artifact quality so they are always first on the equipment list - so I can never forget that they're there.