u can probably use any good pound/buttercake recipe.
I'm going with this recipe for my second attempt at cake carving:
http://www.cakecentral.com/cake_recipe-1972-1-Durable-Cake-for-3D-and-Wedding-Cakes.html
My first came out terrible for the very reason you said...soft cake + soft filling + carving = disintegrated cake. 
For the carved cake I have done I just use my DH mix with a box of pudding and an extra egg. I have never had problems. The thing I do though is to freeze each layer completly before even touching with a knife.
First I wrap and freeze overnight. Then day of decorating, Start assembling and about 30 mins after out of the freezer start carving. At this time it is still a little stiff but the edges are a little softer to cut.
HTH
Can't wait to see what your doing.
JouJ - I agree with leily - after you have taken your cake out of the freezer wait about 1/2 hour then start carving the cake while it is still a little frozen. Adding pudding to your mix and an extra egg also gives the cake a bit more density and will help prevent breaking and crumbling.
Good luck - anxious to see your pics!!! 
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