I'm trying to make a feat for my campaign but I'm unable to figure out how to make it so the armor class of whoever uses it cannot by modified above or below their base AC, for example keeping a players AC (10 + 6 Dex) as 16 no matter what they wear or equip, including from spells. this should hopefully be able to work with something like a monks wisdom + dex increase as well, or whatever the barbarian has as well. is there anything that'll let me make this. here is the description for the feat

"Your armor class cannot be modified by any means, including external effects such as slow or mage armor, and saving throws cannot be altered by advantage or disadvantage, neither can enemy attack rolls. silvery barbs has no effect. This also means armor and other effects can't increase your AC."

Two things:

1. There is an entire homebrew forum for questions such as this: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules).
2. What you want to accomplish is not actually possible. The player will have to override their AC directly on their character sheet to the correct set number.

Instead of using a feat, I think you would be best served by just setting the AC of the character.

Click on their armor class. Then click on customize. Use the option to override the Armor Class.