I've used boxed cake mixes before, and covered them in fondant, but the cake was too soft. The fondant "crushed" the cake and it shifted/changed shape. What is the best type of cake to use when you want to cover in fondant?
I'm going to guess that a lot of people on here will suggest you use the WASC recipe that you can find in here. But I've never had problem with any kind of cake under fondant. I've used box mixes, scratch mixes, never had a problem with any of them. I wonder if you might want to let your box mix bake for just a couple minutes more. I also let my cakes sit overnight before frosting and decorating. They really are much more ready for every part of decorating.
I've used box mixes, doctored box mixes, and scratch, and never had a problem. I only use Michele Foster's fondant, but I've done it in white chocolate, regular chocolate and vanilla.
You can, for sure, try a more involved recipe, but you can also just try adding a box of pudding to the cake when mixing it.
Also try to have a gentle hand when smoothing the fondant on the cake.