this style of punch works well on thin metal and you can change out the head for different hole sizes.
Just did a quick Amazon search, not neccesarily recommending this exact one.
You can use these with LC (I do), just don't inject through them. Like you said in an earlier post, one SHIP and one of those for a filter. If you're really interested in making your own lids, I use these and they are a life saver.
After trying to drill a few I ended up ordering a sheet metal punch that I'm sure I'll use for other things around the house. It was hard drilling through such thin metal and would never be smooth or an even circle.
You could get this kit and not have to swing a hammer at all but I agree with McK, just flip it over for the last step:
Capri Tools CP21050 21050 Metal Hole Punch Set With Puncher 16 Piece, Black https://www.amazon.com/dp/B00OM4GVX0/ref=cm_sw_r_cp_api_glt_i_X62C918SV4F7YVXPR6BT?_encoding=UTF8&psc=1
Don't cut them off. They provide support. I would put caulk between them and use rivets. Aluminum rivets for aluminum, stainless steel rivets for steel. Use a punch if possible to get closer to the corner.
https://www.amazon.com/dp/B00OM4GVX0/ref=cm_sw_r_cp_apa_i_qdZCFbKPKRBG1