This is a caching proxy, based on apt-cacher-ng.  
Note that it expects a netvm called "tor" - you should change this to whatever netvm you want to use.  
Edit the netvm entry in create.sls

Config files are included, which will work out of the box for Debian,Ubuntu and Fedora.  
The cache and log directories are bind-mounted in /rw in the cacher qube.

Copy directory to /srv/salt, then run:  
qubesctl state.apply cacher.create.
qubesctl --skip-dom0 --targets=template-cacher state.apply cacher.install  
qubesctl --skip-dom0 --targets=cacher state.apply cacher.configure

To automatically use the proxy run:  
qubesctl state.apply cacher.use  
This will configure /etc/qubes-rpc/policy/qubes.UpdatesProxy to use the cachingproxy be default.

apt-cacher-ng will cache HTTPS requests if you change https:// to http://HTTPS/// in repo source lists.
To configure the templates to use the proxyin this way, run:  
qubesctl --skip-dom0 --targets=Templates state.apply cacher.change_templates.sls  
Or target individual templates, as you wish.

N.B
apt-cacher-ng works well for Debian,Ubuntu,and Arch.  
It works reasonably well for Fedora, but may require tweaking of the apt-cacher-ng control file.