Basecamp doesn't have built-in functionality to create reusable lists, but here is a workaround that I'm using.

Create a new project and call it something like "List Templates".

Since this project is only going to be used for having list templates, I will start by disabling all other tools "To-dos". Click on the three dots in the top right, and then on "Change tools".

Turn all tools off, except for "To-dos".

Create a new list and add all the items that you want to have in it.

Now when you need a new copy of this list, you just open it up, click on the three dots in the top right, and then choose "Copy". You can now choose which projects you want to duplicate this list into.