Inbox by Gmail - making a small application
You can try Fluid -- it can create one off, WebKit "applications" that look a bit like native applications though are actually single web pages running in a de-decorated WebKit browser window. I've used it successfully in the past for things like Fogbugz.

Fluid may not work though since Inbox requires Chrome and I'm not sure Fluid will identify the browser in a way that'll let Inbox work. It may complain it's not running in Chrome.

If that's the case, here is a shell script that will create a standalone Chrome "app" for you similar to how Fluid works, but with the underlying engine for the app being Chrome. I haven't it used it, but the idea and implementation look sound. You should fork that gist if you're going to use -- gists can disappear. The script, recreated here, for reference is:

#!/bin/sh

echo "What should the Application be called (no spaces allowed e.g. GCal)?"
read inputline
name="$inputline"

echo "What is the url (e.g. https://www.google.com/calendar/render)?"
read inputline
url="$inputline"

echo "What is the full path to the icon (e.g. /Users/username/Desktop/icon.png)?"
read inputline
icon="$inputline"

chromePath="/Applications/Google Chrome.app/Contents/MacOS/Google Chrome"
appRoot="/Applications"

# various paths used when creating the app
resourcePath="$appRoot/$name.app/Contents/Resources"
execPath="$appRoot/$name.app/Contents/MacOS" 
profilePath="$appRoot/$name.app/Contents/Profile"
plistPath="$appRoot/$name.app/Contents/Info.plist"

# make the directories
mkdir -p  "$resourcePath" "$execPath" "$profilePath"

# convert the icon and copy into Resources
if [ -f "$icon" ] ; then
    sips -s format tiff "$icon" --out "$resourcePath/icon.tiff" --resampleHeightWidth 128 128 &gt;&amp; /dev/null
    tiff2icns -noLarge "$resourcePath/icon.tiff" &gt;&amp; /dev/null
fi

# create the executable
cat &gt;"$execPath/$name" &lt;&lt;EOF
#!/bin/sh
exec "$chromePath"  --app="$url" --user-data-dir="$profilePath" "\$@"
EOF
chmod +x "$execPath/$name"

# create the Info.plist 
cat &gt; "$plistPath" &lt;&lt;EOF
&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" â€œhttp://www.apple.com/DTDs/PropertyList-1.0.dtdâ€&gt;
&lt;plist version=â€1.0â€³&gt;
&lt;dict&gt;
&lt;key&gt;CFBundleExecutable&lt;/key&gt;
&lt;string&gt;$name&lt;/string&gt;
&lt;key&gt;CFBundleIconFile&lt;/key&gt;
&lt;string&gt;icon&lt;/string&gt;
&lt;/dict&gt;
&lt;/plist&gt;
EOF