Electrical – How to replace a porch light that’s mounted to a surface-mounted junction box

Those cheap Chinese lights need to be thought out better. There is no one stock part to simply mount them to a proper box. You'll need to make something up on your own. Some of the ones I've seen at the supply house have been put together with a plate from the manufacturer with a rubber sealed connector already installed.

You'll need to mount the bracket to a gasketed weatherproof blank place, then knock out or drill a hole for a rubber cord connector of the proper size to pass the cord through the metal plate.

If you do not have the light yet my strong recommendation would be to find one that is already set up to mount to a box properly.

A tricky situation. If you can't get a piece of texture matching siding that you can paint to match, then I'd go with Aluminum flashing that you can easily paint & profile to the siding. You'll want to caulk & then screw or pop rivet the siding or flashing chunk to just under the siding's closest above rib to act as a roof for your patch, you're screwing the patch into just the siding & not the building or sheathing. First though, screw in an oval piece of plywood to the building's sheathing to fit in the old hole & fill-up the recess to mount your patch & new fixture to, center drilled-out for wiring. Use aluminum screws or rivets to avoid rusting, caulk your patch to the siding & caulk your fixture to your patch. Also, screw plugs with plumbing tape into the new box's unused holes, even if it's a bottom one.