These are two solutions that worked for me. They are by no means the only solutions.
A server (a Dell SC440), with two physical network interfaces. Our office has two ISP connections (DSL broadband). We have the one internal network range (192.168.1.0/24) with two gateways (1.254 and 1.221).
I have installed Promox on the server. I wanted to be able to choose which gateway the guest containers used, instead of them being locked into using the same gateway as the host server. Some of the guest OS containers will host services via the first ISP, and the others will host services via the second ISP. (obviously with firewall rules etc configured in our gateways). So the container gateway entries need to point to their respective ISP/Gateway.
Note: While the security advantage of venet over veth is certainly worthwhile in a hosting environment (and others as well), in my case, we have two hardware firewalls that haven’t let us down yet, and there is little to no concern for the possibility of an internal hacker (there are only three of us!)