Create / Inject JavaScript that turns configuration and url parameters into JS code.

+++ b/customers_canvas.js
@@ -0,0 +1,13 @@
+(function () {
...
+})();

Pass Drupal and possibly jQuery into the js, Then alias them as Drupal and $ inside the script. It makes for easier debugging of JS later on. See https://www.drupal.org/docs/7/api/javascript-api/managing-javascript-in-... as an example.

+++ b/customers_canvas.module
@@ -139,3 +139,37 @@ function customers_canvas_builder($user_id, $entity_id, $entity_type) {
+function customers_canvas_preprocess_page($variables) {

The recommended way to add JS in D7 is via hook_page_build and add it as an #attached element. I think you can do the path checking in there instead. Or alternatively, attach it to the theme element rendered on the specific menu path.

+++ b/customers_canvas.module
@@ -139,3 +139,37 @@ function customers_canvas_builder($user_id, $entity_id, $entity_type) {
+  // Only proceed if we're remotely sure the path is right.

Why? what if someone wants to print things and they are using drush (which doesn't have any urls) or in a test path. This would essentially block that.

+++ b/customers_canvas.api.php
@@ -0,0 +1,44 @@
+  if ($data['args']['owner_id'] == NULL) {

This is weird. Can we check for empty or at least check for === for NULL?

+++ b/customers_canvas.api.php
@@ -0,0 +1,44 @@
+    $current_node = menu_get_object("node");
...
+    $data['args']['entity_type'] = "node";

single quotes more typical.

+++ b/customers_canvas.js
@@ -0,0 +1,16 @@
+(function ($) {

Pass Drupal in as well. It would look like:

(function ($, Drupal) {
    "use strict";
    // logic here.
})(jQuery, Drupal);

+++ b/customers_canvas.js
@@ -0,0 +1,16 @@
+      $('#cc_wrapper', context).once('customersCanvas', function () {

+++ b/customers_canvas_builder.tpl.php
@@ -10,5 +10,5 @@
+<div class="customers_canvas" id="cc_wrapper">

This tightly couples the theme layer to your js. And means the builder can only be printed a single time on a page. If that is acceptable, then proceed.