a 15/16" open end wrench fits mine, and I get it snug but not that snug(don't want nothing coming loose under there at 75 mph) The 15/16" has a 1" on the other open end which I use to fit a large combination wrench into. Lined up with the tranny popping it loose is a piece of cake, and putting it back only use the shorter 15/16" to snug it back up, firmly but not at all aggressively
actually I firm it up and then gently pop it a couple times with the palm to secure it, emphasis on "gently pop it".
It still breaks loose easily with the two wrenches, being lined up is key. If those get set in there too well, trying to break it loose while laying sideways can be bad on, well,, the side
When I get some batteries charged for the camera I'll show you my favorite alternatives for filling, one is simply pull the four screws from the shifter boot, the locking bolt from the shifter, the three screws from the tranny dust cover, and slip the shift lever out(special note to the shift lever, it has three slots, two go up and down the side and slide into the two pins in the neck as a fulcrum for the shifter, and one slot goes cross-ways on the right side, and holds the shifter locking bolt , after loosening the nut to top of the threads tap it out with a small hammer or medium wrech, they come out easy and go back the same way. There are also a couple little shim/seals/spring ring(nothing complicated here, once you see it you'll understand) but those will ride out on the shifter and go back the same way'
In spite of the initial removing/learning process the second time you can do it all in probly 15 minutes(beats drinking/bathing in fluid while trying to go into the side filler until figuring out that method. Other guys have other methods also which are good but I really haven't any of them myself. When I can get a picture I'll show the little fluid bottle and hose attachment I also found to work, altho not exactly flawlessly, from beneath
Near forgot, here's the little oil filler tube I last used 12 years ago. It came with a longer hose which I shortened to make it easier to fill the transmission, just hold the bottle upright and twist the end of the hose into the filler socket and then ease the bottle up until it goes in. That yellow thing also twists to shut and open a valve. It worked ok I guess but thinking there was some leakage. I also think it fits the rear axle oil bottles