Fish farms are by and large the easiest and most compact way to get XP any where in the world with little trouble. Other than that I would say feed a zero tick cactus/Bamboo farm into a furnace and have it store up Xp. It could always be running and when you need it you just remove the dye. Far as I know there is no limit to how much XP can back up in a single furnace but it can be fairly slow to build up if you don't have multiple going at once.
Don't have a link to an exact build right now though but they are pretty common. If you don't care about the exact amount of Xp you get they are really simple though. I tend to just have the furnace lead into a chest/double chest. When it's full of green dye and the furnace backs up you can remove it for tons of XP.