The slipcase is very cool!
While we are on the subject, here is a quick update on upcoming covers. The "basic" 2nd printing will have the existing "blue" cover. There will also be two alternate covers. Doug had some ideas for a very cool painted cover, which will be the first alternate cover. It will be available in late October. The second alternate cover for the 2nd printing is being painted by a TSR veteran. It will be available later in the year, probably December. All of these covers will have the same "guts" as the basic 2nd printing, just different covers.
I had considered a slipcase for one of the alternate covers. To do as you have done - leaving space for other books - is difficult from a logistics perspective. It is hard to ship without being crushed or damaged. A "tight" slipcase that encloses only the book it comes with is more feasible (but expensive).
As an aside, DCC #71: The 13th Skull also has a silver foil alternate cover, which I will post soon. I originally titled this adventure The Silver Skull (as those of you who have played it will understand) and a silver-foil cover featuring a skull seemed appropriate. The third printing of The Dungeon Alphabet will also have an alternate cover, printed in gold foil by Stefan Poag, which turned out great.
This is great news. I can't wait to see the new covers.