It is true that it is difficult to find Open Source projects that provide a real Squid Web administration interface
Here it is a real Open Source project that should provide this topic.

Artica For Squid is a user-friendly Web ajax administration console that try to provide all necessaries needs in a company.

Users/Categories/Web sites statistics and for several years
Proxy.pac full administration.
Url filtering with more than 6.500.000 categorized websites.
Proxy monitoring
ACLs management by UserAgents, MAC addresses.
Time management
bandwidth management

There are several other features...

How to use it:
You need a Free system :

You can install Ubuntu yourself and use the installer binary
located on the Artica Project Webiste

You can also download the ISO (both 32/64 bits) on SourceForge


To obtain more informations and for support needs, there is a dedicated Forum: on Artica project Website

Execuse, me cannot post any link here...