[Discuss] Simple, unintrusive firewall solution?

[dbarrett at blazemonger.com (Daniel Barrett)]

tl;dr: Seeking help preventing one network from seeing another.

I have two routers -- call them R1 and R2 -- and would like to define a
firewall rule so clients of R2 can't see clients of R1. Something like:

  iptables -A INPUT -s <R2's network> -d <any R1 client> -j DROP

Unfortunately, neither router allows such rules to be defined. (R1 is
a Verizon FIOS Quantum Gateway serving a wired network. R2 is a Google
Wi-Fi mesh router connected as a node on the Verizon wired network.)

Any suggestions how to achieve this goal? My first thought is to place
a simple hardware device between the two routers. In my dreams, it's a
tiny, low-cost box with a WAN & LAN connector and running iptables, and fast
enough that it doesn't slow down R2's network.

The firewall market is huge, and I've been reading reading device
manuals on & off for several days. A recommendation would really help!

Or is there a better solution to my dilemma?

Thank you very much for any insights.

--
Dan Barrett