Just wanted to throw in I wouldn't fill it. Otherwise you could have the top of whatever you are carving slide off, LOL. Or if you fill it you could put a straw down the middle to hold it all together.
I am making a small car cake this weekend and have been trying to decide if I should risk filling and using a straw to try to keep it together. Mine won't be very tall so I might risk it.
Well, I use buttercream to fill cakes that I carve, especially topsy turvy type cakes and such. I would avoid some of the other fillings like pudding and such because they are not as stable. But I find the buttercream acts more like a glue and since I always use a stiff buttercream dam before filling even with a more thinned down buttercream, I haven't had a problem.
I use pound or madeira cake; bake, freeze it before carving. Yes, I fill it and freeze it again. If it becomes soft while carving I put it back into the freezer to get stable again. These cakes hold well for 3D cakes and fondant use. If you do not want to use a "from scratch recipe" then try pound cake mix and follow instructions in the box. Do not try to alter the formula because it could affect the consistency of your cake. Let us know how did it come out.