Hello Davelant,
the best program for designing and unfolding in Linux is Blender 3D.
Just google about it. I know, Blender is discussed quite controversial.
Some really hate it, because on the first view it is quite tricky, but if you have gotten familiar with it, you'll love. It'll take some time to learn how to use it, but afterwards you won use any other program. And it's absolute free, no fee at all!!!!!
for unfolding you can use B-paperizer.script, you download it (just google it) and copy it in in the script directory of Blender.
With this script you can also export the unfolded meshes as svg.files, so that you can import them into Corel, or better in Inkscape or Gimp, as you us Linux!
Thereś also a free book out in the net, about designing cardboardmodels with Blender.
Here is the link:
Free Paper Model Design eBook2 by ~PixelOz on deviantART
There is all you want to know about Designing with Blender, and there is also the B-Paperizer script .
I hope i could help you .
Good Luck
The Oldenburger