Just saw that this thread has no replies yet and thought I would chime in, being that I just purchased the same sub and am currently having a box built for it as well. IMO, anyone's best bet is to go with what is recommended on the Image Dynamics website. You can adjust the size by 10-20% based on your preferences. Just do your research on how changing box sizes changes the typical response curve of a subwoofer, then do the same research specific to your subwoofer. From what I have read, the IDMax is not a very finicky subwoofer and does well in a range of applications and box sizes.
I just scored a brand new IDMax15v4 as well (for $350 shipped) and am in the process of building a box. However, I am going to build a rather large sealed box, about 3.3 cubes. This sub does not need a box that big, but the larger box should help bring out the low end a bit more, while still giving that tight sealed accuracy. I bought a JL Audio 13w7ae and a JL Audio HD1200/1 off of a guy for $750 (a steal, I know), and I had a 4 cube box built for it, ported at 34 hz. Been running that set up for a good year. I love the output, and while it actually sounds pretty tight... it's not as tight or accurate in the low end as the 2 sealed 12w6v3s I sold when I bought the 13w7ae. I am going to keep both the sealed IDMax15 and ported 13w7ae to interchange at will between sealed and ported sound, and I would recommend that anyone with the budget to do so do that also.
Ok. Sorry I got off on a tangent there... back to your question. In my experience small sealed boxes do just fine. But adding a bit of space helps the sub breath a bit more and gives a touch better low end performance in a sealed capacity. This is just my humble opinion and this opinion may vary from person to person. Ultimately, I recommend this: Try going for around 2.5 cubes sealed on the IDMax15v4 for an all around SQ build, and I would say the wattage you have is just fine, being that it is a pretty efficient driver. I doubt you will be unsatisfied. Hope to hear how things go.