I just measure the size of the cover needed and create a box for the extreme edge of the artwork, then design the cover using the software. And for CD labels (if needed) I create a template in the same software using some vector circles and then create the label in the same way. Has worked perfectly for me for years.
Ended up with a similar solution. I opened a blank template in that otherwise useless Disketch program I mentioned, and took screenshots of the templates which I then pasted into MS Paint and used to design the whole thing there. Imported the finished images back into Disketch which after all was able to print them out in the right size. Worked out fine for me, of course it's not an ideal solution if you'll be doing this a lot but like I said I only needed to make this one cover.