Backstory: There are some gaps around my apartment doors I'd like to fill, most notably a diagonal gap at the top of the main door, as shown in the photo. I own the apartment, not just rent. Replacing/restructuring the door/frame entirely for a perfect fit would be the ideal solution but meanwhile I want a quick fix to cover the gap, primarily to prevent the smell of secondhand smoke from smoking tenants entering my apartment, which occurs regularly as they smoke at the front door of the building close to my apartment. Any sort of padding I place in this gap such as weatherstrips fall out with continued use of the door. I wanted to screw on an upside-down door sweep but the metal door is not very accepting of my cobalt drill bits, as you may be able to see in the photo.

TLDR: How would you fill/cover this gap? Weatherstrips fall out and door is unfriendly to drilling.

 

The gap is there because the frames are out of square. Fixing it would likely mean a new door unless you can stick a pry bar underneath the bottom of the hinge side jamb and raise it up. Not sure how your jambs are fastened, but that's what needs to happen to square the door. They might also be stuck to the wall by years of paint and caulk, so I doubt it will be easy to raise the jamb.

Your idea of installing a sweep upside down might help some. You will likely need a better drill bit, on low speed. You can also use self tapping screws, but you will still want to drill a pilot hole for them. Better weatherstripping could be applied to the outside of the door on the stop. Aluminum / bulb style weatherstrip. It fastens to the door stop with screws, so you would need to predrill and screw it on as well.

Aside from that idea, the only other thing you could do is build up the top of the door with a shim, cut from wood to the length, width, and tapered shape you need... sealed to the top of the door with silicone or similar.

 

Attach a piece of wood with foam weather stripping against the door to the stop/frame on the other side of the door to cover the gap. If you can't drill into the stop/frame, use construction adhesive to fasten the wood strip.

 

Managed to drill it after applying 3-in-1 oil. Metal layer was thin, no more than 1cm. Screwed on an upside-down sweep over the top gap and self-stick sweeps over gaps on the side and bottom of the door. So that's my current DIY project done.

I guess for my next project relating to sealing my apartment I'll eventually get to properly covering/filling the gap at the top of the front window which doesn't close all the way, at least until the closing issue is fixed - although I'd have to move the coaxial cable to another window to allow this window to close fully as the cable comes down from the building roof entering my apartment through this window. Right now I'm simply using an inordinate amount of duct tape over the window gap which nonetheless provides some degree of insulation and which is easily taped snugly around the coaxial cable. Nonetheless any tips on proper fixes for this are also welcome.

In the meantime, thanks for the suggestions. Really appreciated.