I have settled upon the following - it is the cheapest alternative. You can buy custom made bags which keep the obi separate to the mini but I have not tried these and they may not overcome the problem of being able to view the spine.
I should do (and will do) some scans to illustrate - but for the time being....
In effect I simply turn the bag on its side (flap facing up - top opening in other words), fold the obi over the bottom of the cover (not the spine) and then load the CD/obi into the top of the bag.
In other words, the obi sits at the bottom of the bag with the base of the cover sitting inside the obi. This also means the flap folds over the top of the back (rather than the back of the back) so that when you push the package back into the shelf it does not hit and fold over the neighbouring mini LP.
This only works for minis (not digipaks etc) because they are square shaped