That's one I've thought about, and I came up with two possible options. Unfortunately both cost money.
The big problem in my mind is that if you open ports it's almost certain someone is going to get in eventually. The best solution is keep the ERA system isolated from your own network.
One option is to create a DMZ on the home network using two firewall routers, and then host ERA on an old system attached to the outward facing firewall with the appropriate rules to publish in place. This can be done with old equipment if you have it. The downside is if someone compromises that system they will probably be able to monitor everything that goes out to the internet from inside your network.
The other option is to use a cloud hosted vm. It wouldn't really need to be very powerful, and it would keep the risk external to your own network. You can get them through game hosting services, but it's going to have a monthly fee, and even a low powered VM may be $20 - $30 a month. ERA can run on Linux, so you might be able to do it for less as long as you are ok working with a NIX OS.
There are cloud hosting services that only charge you for the VMs up-time, but I can't give you any recommendations for a service.
Either option could also be used to host your virtual tabletop software if you use one, but AFAIK Fantasy Grounds is windows / mac only. Any service that offers windows VMs will likely charge you a fee for the windows license.
Personally I prefer the cloud host solution since it keeps it completely separate from the home network. A hosted VM can be easily re-initialized, and ERA is like a max 5 minute install even with all the books, and any customization files.