Template Selectbox Field

Installation

Upload the 'templateselectboxfield' folder in this archive to your Symphony 'extensions' folder.
Enable it by selecting the "Field: Template Select Box", choose Enable from the with-selected menu, then click Apply.
The field will be available in the list when creating a Section.

Create a new folder within your workspace for your custom templates.
Create your XSLT files and place them in the new folder. You can optionally give your templates a user-friendly name by adding a comment to the top of your file as follows:
Adding this field to a section and select your new template folder.
When editing/creating entries in your new section, you will now see a 'Page Template' field where you can select your custom templates. Note: By default the existing page template is used.
Attach the field to the main datasource you are using for retrieving entries from your section (note: only 1 page template field can be used per Symphony Page).

I used Nick Dunn's & Brendan Abbott's Upload Select Box Field as a starting point for this extension.