This is something I'd like to do as well. The custom post type permalinks plugin does not work to change the "otw-portfolio" in the URL. The plugin only allows you to change the slug AFTER otw-portfolio. So your URL still looks like mysite.com/otw-portfolio/CUSTOMIZABLE-SLUG. Which is redundant because you can already change that slug in the portfolio options.
The theme demo of your theme Carbon that uses the portfolio plugin does not have the "otw-portfolio" in the URL. As seen in this URL:
Then again when clicking on an actual portfolio item:
I've tried changing some stuff in the php files to change the custom post name but it doesn't work properly.
I would really like to use this plugin but unless I can change the otw-portfolio in the URL I won't be using it. Could you not provide the customized files similar or the same as the ones in your demo? Or explain which files need to be edited...there are a lot of otw-portfolio instances in the few php files with the plugin.