Held together.
Now, what I could do here is one of two things :
1: Glue the bottom to the top, and continue putting filler on there, then take a cast of them both together, which will then be used to mould everything, and then put the back on later.
2: keep them separate, and then use them to vacuform the top and bottom separately, which will then be glued to a centrepiece and back made of thick plastic.
I'm thinking of the second, because I've made enough strange smells of epoxy for now, plus I can make two pulls, one of thicker plastic as the main body, then one thinner which can be cut up and used for panelling.