If you have space in the heel - that is a major issue, as you lose a lot of control over the skate. You can cut a new insole that wraps around the foot, to somewhat fix it, and you can do the same on the sides - but it would be hard to deal with the whole foot. You could try an extra thick insole to push the foot up into the smaller section of the boot (e.g., that 3 insole solution). That would also push the foot back so the heel lies against the back of the boot, which is desirable.
Incidentally, if you got longer blades to match the longer than needed boots, you could have a separate problem. If the fitter positioned the blades so that the sweet spot is too far from the ball of your foot, it would be difficult to control your skates. And if he/she positioned it right for that - your toe pick may be too far away. And the tail may be so far back that you accidentally step on it once in a while.
A better solution is of course to spend the money and get the right size. A good rule of thumb, for US sizing, which I believe Riedell uses, is the one I cited - 1/2 size up is 1/6 inch in length. And proportionately larger in width. I would take out the insole and see how your foot fits on it. If 1/6" smaller in length and <current width> / <current length> * 1/6" smaller in width would do what you want, that is about right. Of course, the insole (which approximately reflects the footbed) isn't the whole story, but it may give you a rough estimate.
For example, here is one approximate set of measures, that isn't specific to Riedell:
http://www.galls.com/photos/static/documents/galls/pdf/how-to-measure-footwear.pdfAlas it doesn't go down to your size.
But
http://www.shoesize.com/men/shoe-width/sizechart/and
http://www.shoesize.com/women/shoe-width/sizechart/makes it clear that at your size level, one half size down will take you about 1/16 inch narrower.
However, you can order Riedell in more than one width - the approximate difference between B, C, and D is given by about .2" in width.
http://www.bootoutfitters.com/size_chart.htmlRiedell will even let you specify different widths at different points in the boot - though they might charge for the customization.
I hope that wasn't too confusing.
It would be nice if Riedell gave you a better way to figure it out for yourself... Like a set of insole tracings, that you could print out and step on.