I added a metal stud wall but the existing junction boxes seem to be the type for wood (screw holes on the inside at an angle). Can you screw through these into a metal stud or is there an attachment to use with them instead of using a dedicated box for metal studs?

I'm building a small 16x20 remote cabin in Alaska and have a plan in mind to use a two slot square D box and a 10/3 electrical extension cord as the input power to the box. My plan is to cut off the female end of extension cord and run it into the box - using the two hot wires as power lead to two - 20 amp circuit breakers. One breaker will power all electrical outlets (10 are installed) and the other 20 amp breaker to run 9 recessed LED lights and two 4 foot LED shop lights. The male end of the extension cord will plug directly into a 4200watt inverter generator about 20 feet from cabin. I don't plan to have all the outlets in use simultaneously nor all the lights on at once but want the flexibility to plug in small appliance when needed. I'm trying to keep costs down and not mess with RV type 30 amp plugs or receptacles and I figured the extension cord would be simplest and cheapest fix. Am I on track with this idea or would I have to clip one of the incoming hot wires on the 10/3 cable and then run a jumper cable between the two circuit breakers? Any ideas or suggestions appreciated.