Solution: Rice Krispie squares (or "cereal treats" - hopefully I'm not risking a lawsuit by using a trademarked/copyrighted name) - a solution to short cakes and many of life's other problems. Not only do they function as delicious cake implants (who doesn't like Rice Krispie Squares?), but they also provide a strong base for wood dowels that were inserted into the 8" layers to support the 6" layer on top.
All you need to do is follow any regular Rice Krispie square recipe, and then pack the mixture into whatever cake pan you used to bake the other cake layers. When you're done, you can just stack the cake layers right on top.
A few tips:
1) Cakes tend to shrink away from the sides of the pans when they bake, whereas Rice Krispies don't experience any sort of shrinkage. Ergo, you will experience a discrepancy in size between the cake and the base. Make sure that you fully compensate for this when masking the sides of the cake so that your sides are plumb!
2) A light greasing or buttering of the pan may assist with unmolding later on. I didn't use any, and my Rice Krispies came out fine (and I wasn't using a springform pan either).
3) Patience is a virtue - wait until everything has fully cooled before trying to unmold, or you might end up with a warped Rice Krispie base. (I'm not good at waiting when there's a cake to be made, so this is always a challenge).
4) Level, level, level. Make sure the top of your Rice Krispies are as level as possible since you're building the rest of your cake on top.
5) If you're covering the cake with fondant, make sure you ice over the Rice Krispies for a smooth surface. I thought I would be able to get away without icing, but I ended up with a stippled texture showing through the fondant. (Easily hidden with a bottom band, but that's not always desirable).
6) Make sure your Rice Krispies are securely attached to the cake board, either with Royal Icing or softened fondant. You don't want your cake sliding off into the floor at the feet of your [extremely] surprised guests.
True story - not with a cake - but I definitely had prepared a fantastic apple tart for a friend's Birthday, only to gracefully deposit the tart, freshly warmed from the oven, onto the kitchen floor.
A pie with a date - with the floor. Note to self - tart bases are extremely slippery.