The Dungeon Crate is a type of crate that can be obtained via Fishing in the Dungeon. Opening Dungeon Crates is the only way to obtain the Golden Lock Box, and therefore any Dungeon-exclusive items normally found in locked Gold Chests, once the Dungeon has been fully looted.
As the Dungeon does not commonly have any type of lakes for the player to fish in, creating an artificial lake is almost always necessary. A simple, yet effective way to do this is to drain any bodies of water near the Dungeon into it, by digging a tunnel from them to the interior of the Dungeon.
As long as you are below 0 depth and in front of natural Dungeon wall, you are counted as being in the Dungeon. (It is notable that the music changes as well. This is an easy way to tell if you are in the Dungeon.) If you drain nearby lakes or even the Ocean into the Dungeon, you may want to do it in the outer wall of the Dungeon. Fishing here serves for better protection from Dungeon enemies, and they will also be less likely to spawn.
If you are willing to spend a little effort, you can make a perfectly safe (no enemies will spawn) fishing spot at the edge of the Dungeon. Find an isolated Dungeon branch, fill it with Dirt, Hay or any other cheap block, mine a 2x3 niche at the outer edge to expose the Dungeon wall, then create a lake nearby. If you then stand in the niche, no Dungeon enemies will spawn because there's no free space for them nearby, and no other enemies will spawn either because you're technically in the Dungeon. So, you can fish Dungeon Crates in peace.
For bonus points, you can place some mud nearby and plant Jungle grass on it to be able to catch Underground Jungle-specific fish (for Summoning Potion). This will make regular cave fish more rare, though. It is also possible to infect the surroundings with Hallow, Corruption or Crimson for a similar effect, but you will no longer be able to catch Dungeon Crates in that case. Enemies will still not spawn, however, so it may be worthwhile.