How to add a new sidebox from scratch?

We are trying to add a new side box for a customization for a client. I want to make sure we do it right instead of just hacking it.

Here is what I can gather:

add a file in /includes/modules/sideboxes/custom_sidebox_name.php
This is the actual meat that has the functioning code.

and then add a file in /includes/templates/template_default/sideboxes/tpl_custom_sidebox_name.php
This is the code for how the box looks- it looks like this is actually required in the preceding file?

I'm not worried about a defining language file because the entire site is exclusively English.

Am I even in the right ballpark?

Also, what actually makes the file appear in the sidebox controller so it can be turned on and controlled in the admin?

Thank you in advance for any help!
:)
Jenn

Am I even in the right ballpark?

Also, what actually makes the file appear in the sidebox controller so it can be turned on and controlled in the admin?

You're hitting a home run.

The sideboxes are auto-magic and as long as you have the right files in the right places, The Zen Cart Admin will find them.

You could also try the Blank Sidebox mod which puts everything where it belongs. Works great for me.

Thank you so much, guys! Zencart can be a little overwhelming just to dive in and find everything- but I'm slowly finding my way. :)