How to create a custom GET include

I am building a custom theme, and there is a sizable chunk of code that is repeated on some of the templates (but not all). I want to create a custom <?php get_toolbox(); ?> file.

1. is this possible?
and 2. can anyone point me in the right direction?

I thought about building a widget area, but it would be more complicated than a simple include on the templates I want it on vs. a custom plugin, custom widget area, additional plugin to set which pages it appears on, etc.

Thank you for any help!