I used these:
Whiteside 6700A Slotting Cutter... https://www.amazon.com/dp/B01N7JEXIT?ref=ppx_pop_mob_ap_share
Whiteside Router Bits A200B... https://www.amazon.com/dp/B0012JI6M2?ref=ppx_pop_mob_ap_share
In a trim router, I was working with plywood though.