I am making a rectangular building cake for a family birthday where I will be traveling to get there. So, I will not have a lot of time to decorate when I get there. I am thinking about making the walls out of gumpaste or a gumpaste/fondant mixture ahead of time and then attaching them to a buttercream frosted cake when I get there. I am thinking that we can remove some of the walls in order to serve the cake. Any thoughts on this?

Pastillage is sturdier for walls.