3500W generator I picked up

What you are planning IS NOT LEGAL and could get you into trouble if "they show up." Me? I would do it in a heartbeat as long as no family members have any possibility of messing with the breakers, and you are "fully cognizant" and so on.

You are supposed to have this set up, either with a transfer panel or a mechanical lockout so that none of this can be interconnected

I got one of the lockout mechanical kits for my main box. There is no way to do such a thing from a sub panel