I got the same set up. I had a coffin for it, Odyssey brand, but found it too heavy to move around. So I went with individual cases. Again by Odyssey.
Odyssey Model numbers are: FR12MIXE for the DNX1600 mixer and 2 FRCDJE for the Denon 3700s. Got em on eBay. Cost me less than $300 for all three.
Having the individual cases make it easier to fit in the trunk of my car too and easier to lift on and off the DJ booths at the bars and clubs I dj at.