I definitely reccomend making your own, esspecially for a convention game, ESSPECIALLY for a first time DM. Here's a recipe book for you to use. It's cliched, but that's what makes it so great for a first time game, actually.
Think of a cool toy. A magic item. A McGuffin. Something that PCs would want.
Think of a guardian for that magic item. This guardian should be more powerful than the PCs, but still defeatable. Something that will give the PCs a good challenge.
Think of a puzzle. For puzzle making, don't concentrate on the puzzle aspect, make up the solution. Say the solution is "Go to point A, then point B, then point C". Now giving the PCs a reason to do that is simple. Could be anything. Perhaps you need three keys (the DooM route) or perhaps you're tracking something, or maybe you need three mini-McGuffins that will give you a fighting chance against BBEG (maybe make the BBEG a largish red dragon, then have the mini-McGuffin give you fire resistance and immunity to fear and some bonuses to get through all that armor). Something. Just come up with the solution to the puzzle before you come up with the puzzle.
Now draw a map. Since you've already thought up a puzzle, you probably already have a good idea how your map should look.
Now put monsters in. Monsters should probably guard all the key locations of your map... the puzzle answers, crossroads, that sort of thing.
Think of a finale. This could be the recovery of the big McGuffin... or it could be what happens afterwards.
Now you have a nice little dungeon you made yourself! You know everything about this dungeon, you can modify it on the fly, it is YOURS and yours to command.
Here's an example I came up with in these few moments...
Party ECL: 5
McGuffin: Ring of Three Wishes (cliche'd, but thats what we're going for)
Guardian: Clay Guardian (CR10, ouch!)
Puzzle Reward: Amulet of Magic Penetration (made up item that enables the bearer to ignore spell resistance/immunity) and an Adamantine Sword (maybe more than one)
Puzzle Solution: Talk to NPC1, Go to location A and retrieve thing Z, give thing Z to NPC2, go to location B and meet NPC3, goto location C and kill Monster D
Finale: On touching the ring, the dungeon starts to collapse as Animate Object is cast on all the carved support pillars of the dungeons. (The support pillars are carved to look like people... ancient kings perhaps. They come to life (use animated object stats) and the roof starts caving in, AND THEY START FIGHTING THE PCs!)
You can use that as a starting point if you like. If not, I'm sure you can come up with something that suits your needs just fine. It isn't hard at all, and it's very rewarding.