In my old darkroom, I used a solid core steel exterior door with a magnetic weatherproof gasket around the perimeter and an aluminum threshold. It worked well.
My new darkroom has a wood interior door with weather stripping along the jamb on 3 sides. A rubber door sweep on the bottom blocks out alot of light. On the exterior, I nailed up some rubber gasket material that usually goes on the exterior of a garage door and that helps alot. I still get a few leaks at the lower corners so I just put a couple of Jobo drums there to block the light.
I found that on sunny days I get leaks through electrical outlets because there is an outlet in the same spot in the adjacent room. I taped a piece of mat board over the outlet in the other room and that solved the problem.
I have also found light leaks along the baseboards if the adjacent room is sunny. I did not want to do anything elaborate so I rolled up some old T-shirts and put them along the baseboard in the sunny room.
I use black electrical tape to cover miscellaneous LED's.
Island Heights, NJ, but will retire back to Maine.
Med. Format RF
All of my darkrooms have been in basements or attic storerooms where heat, water, floor joists, and sewer pipes have been running through, thus allowing for multiple sites where light can get in. I've discovered over the years, that absolute light-tightness is not necessary. The trick is getting it to be light-tight where needed and then not worrying about the rest. For 4X5 tray developing, which is the only kind I do, I always have the developing trays in the sink (which has a six-inch side), so the film and trays are blocked from any light leaks coming under or around the door by the sides of the sink. If the light leaks are down low, there's little to worry about. If they're above sink level and overlooking the sink, then they need attention. I've discovered that black felt, wall joint compound, and flat black paint are wonderful things. Wherever I've got a leak that can't be 100% sealed off because of weird-shaped openings or pipes, etc., I paint around it with flat black paint which, if the leak is a very small one, is often sufficient to neutralize it. The only fogging problem I've ever had due to extraneous light came from developing my sheet film beneath a too-bright Gralab timer, which I now cover with a cloth.
I've been pretty quiet while each of you have been kind enough to post your ideas. I am in the process of implementing some of them. And, Chazzy ... I find all of these ideas to be pretty 'eye-opening'....