Back then, I put OpenBSD on a spare P133 with two network adapters to do something like this: https://www.openbsd.org/faq/pf/example1.html

VM and/or containers make sense today. I've got an old Mac Pro tower I found on Craigslist in the basement. 64G of RAM for it was less than $150.

Since Cisco offers virtual versions of their routers, you can look at what people are doing to build home labs for practicing for Cisco certifications.

The homelab subreddit has a lot of interesting examples of peoples' setups as well.