As the title suggests, I'm trying to create an item that increases the save DC for arcane shots, but none of the options look right. I tried adding a bonus to spell save DC but that doesn't work, does anyone know where I should be looking or if it's even possible? There doesn't seem to be an option under arcane archer to manually adjust the arcane shot dc either.

There is not an option under the Archer, nor is there a way to do it otherwise. That DC is calculated something called a Snippet attached directly to the subclass itself. The only way to adjust it is to create a new subclass using the Archer as a template, and then editing the snippet itself.

Alternatively you could include it as plain text and the player can be responsible for remembering to add it themselves mentally.

Thanks for confirming my suspicions and putting it in the splash text and letting the player remember was my back up plan if I couldn't find a way to do it anyway.