IIS URL Rewrite module issue when trying to add trailing slash - asp.net

Question

My company launched a redesigned version of our site this past October. We have a normal ASP.NET domain setup, but some pages were renamed and moved to new locations, so we had to rely on the URL Rewrite module in Server 2012 to ensure that bookmarks were routing to the correct locations.
In most cases, a 1:1 URL match based on regex patterns or a rewrite map works fine and as you would expect. However, we have some legacy directories that I had to move over as well, that weren't a part of the redesign project. I moved these folders into a folder off of the root, something like:
$/sandbox/annualreport/
so that we can monitor and update all of the folders that haven't been redesgined in one place. The abridged rewrite entry I am using is this:
<rewrite>
<rules>
<clear />
<rule name="Sandbox URLs">
<match url="^(annualreport)\b/?(.*)" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false" />
<action type="Rewrite" url="sandbox/{R:1}/{R:2}" />
</rule>
</rules>
</rewrite>
This works fine for all URLs when the directories are followed by a trailing slash, like $/sandbox/annualreport/ but not when the trailing slash is left off, like $/sandbox/annualreport, even though the / is optional. The pages load and they are accessed correctly, but the problem is, some of these directories have local css, images, and scripts, which are accessed in the HTML like href="css/main.css" or src="scripts.js". When I view source and click on those style or js links, the rewrite looks for these files off of the root, unless the trailing slash is added.
This has become a nuisance because I have to have a 301 redirect for every "sandbox URL" (before this rewrite entry) to add the trailing slash. This works, but I have to specify every folder name in the regex:
<rule name="Custom Add Trailing Slash" stopProcessing="false">
<match url="^(annualreport)\b$" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false" />
<action type="Redirect" url="{R:0}/" />
</rule>
I have tried the default URL Rewrite wizard for universally adding a trailing slash, which I would prefer, but it doesn't seem to fix the localized files problem:
<rule name="AddTrailingSlashRule1" stopProcessing="false">
<match url="(.*[^/])$" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false">
<add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
<add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
</conditions>
<action type="Redirect" url="{R:1}/" />
</rule>
How can I force a trailing slash to be added on a URL rewrite while maintaining the locally accessed files within the rewrite directories?
Any help would be greatly appreciated. Thanks.