Either one is simple. All a rat bike is is a busted bike fixed with whatever was handy. Like me having a Ford regulator on a Honda.
A bobber is just as simple: cut off everything you don't want.
This is probably what you want:
I had a 1979 GS425L. This is what was done to it:
Battery, airbox, toolbox removed. Welded 2 pices of angle iron to the frame where the airbox used to be = new battery mount for lawn and garden tractor battery. Added 4 Emgo mesh air filters. Cut the stock seat in 1/2 on a bandsaw, recovered = solo seat. Rebent already bent handlebars to new shape. Flat black exhaust, gloss black engine, black laquer on sheet metal. All paint worn, weathered, and scratched.
Homemade highway bars welded to engine mounts (1x1 sq. tube and 1x1/4 flat).
A true story: I was riding this bike down to the Weirs during a Bike Week. When I was stuck in traffic on the hill, a guy riding a Suzuki GS was next to me. He looks at the bike and says "Hey, is that an old Triumph or something?" I laughed and answered "No, it's just like yours, only a bit smaller and a few years older!" I was glad traffic started to move, my eyes were tearing from laughing so hard.