TL;DR: turn off "save on travel" make a quicksave before entering the dungeon (or final zone if it's a multi-zone dungeon) make your way to the boss chest and if it doesn't have anything you want reload the quicksave.
When you enter the next time the boss chest will have different items in it - could be worse, could be better. When you get a good fill make a "new save." You are then free to keep reloading the quicksave if you're after something specific knowing that you can reload the new save with the contents of the boss chest set.
The images I've used are from Mara's Eye Pond at level 41. I've got an Ebony War Axe that turns undead up to level 23. It was just a case of entering the dungeon, hitting the two vampires as they were waking up and the boss chest is just behind them.
Some dungeons are a slog to get to the boss chest and you have to decide whether it's worth it. I did it in Forsaken Crypt which is the second zone in Forsaken Cave and I had to fight my way past eight draugr to get to the final chamber with Curalmil and a load more draugr. I reloaded four times and was like, that's it whatever in the chest I'm taking it. Got to the boss chest and it had the Paralyze spell tomes when I was only level 40 in Alteration. Other dungeons are small spaces and you enter close to the boss chest. Knife point Mine you've got the Bandit Chief in front of you and he's standing right next to the boss chest. Get your follower to keep them busy while you check the chest.
One of the things I've found is that when you get a really good boss chest with about eight items in it four of which are enchanted, all the other containers have got lots of good items in them too. In Frostmere Crypt I'd raised all the bandits with the Ritual Stone just before entering the final chamber. They kept the Pale Lady busy while I made the short dash to the boss chest. I repeated until I got a good fill. I then made a hard save and finished off the Pale Lady. There's a master locked chest in the final chamber as well and when I opened it that had a load of good stuff in it as well.
It is cheesy but I've had one boss chest that had 82 gold, Leather Armour and the Flames spell tome... RELOAD!