Buddy & Sugarskull: Actually, you're not allowed to edit these models. And I still don't see why you guys go on about it as it won't even be visible once you textured it.. if you turn on doublesided textures it'll just show as black, which such a gap normally would
Anyways, I guess you're allowed to scale the door up a little to cover it. Please not however that normally one ain't allowed to change the models at all.
And as a tip, if you render so close to the box that the gap is clearly visable, then you're either rendering out a very large image, or you zoomed in so far that pretty much nothing of the model is visible.. plus.. it'd just make it look lowres
Sugarskull: While sharpening my textures I just use the standard sharpen filter. And I don't go in sharpening different layers different amounts at the end, I simply make sure they have an even sharpness and look good through the whole texturing process, then sharpen it up a bit extra towards the end. Don't go overboard though, it'll completely ruin the whole texture if used too much.
Cheers!