I've been searching for suitable boxes for this and I found a cute one on wedding star:
http://www.weddingstar.com/product/love-letter-ceremony-box-setIt's a bit pricier than I expected but considering all that it includes I don't think it's too bad. Plus I think the box is really pretty and classy looking, which if you think about it, liking the box is important. I mean that's all you'll be looking at for a long time haha. Anyway it includes two wine glasses (in the pic it looks like it has something on them but I don't know if they come that way), key and lock, stationery for the bride and groom and even example ceremony wording. And it has like a convenient little pocket to put the letters in. I showed it to FI and I think we might be going with this one. He really likes the idea
Even if you don't include it in your ceremony I think it can give a nice touch to any marriage.
Btw date twins, I noticed in one of the pictures of the sample engraving on outside of the box, it has our date!! I thought that was cool! Hmm maybe it was meant to be..? ;-)
**ETA: For those who have no clue what I'm talking about lol:
http://www.celebrateintimateweddings.com/ceremonylovebox.html