For Sitefinity 5.1 the only possible way to map template of dynamic content widgets is the second option from the blog post of Josh Morales.
However, this is valid for Sitefinity <5.4. In 5.4 and 6.0 you can use the first approach by directly entering the path to your aspx file (relative path) in the TemplatePath property of the designer. This way you will not experience the problem that is currently worrying you. I would suggest you to upgrade to Sitefinity 5.4 or 6.0 and take advantage of this option. More information about the upgrading process could be found in our Sitefinity documentation
All the best,
the Telerik team
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