After cutting four rectangles out of the 15mm MDF and one out of the thinner plywood, I glued each of the slabs together using plenty of PVA glue, before gluing on the axe and counter-weight.
For the “slabs” connecting the axe and the counter-weight, I needed a material that could withstand the extra weight of the two halves as well as provide a sturdy base for attaching the handle (otherwise, there was a chance that the axe could break if I swing it around and let’s face it, what’s the point in having your own Stormbreaker if you can’t swing it like a madman?!) Luckily I had some spare scraps of 15mm thick MDF and 10mm thick plywood that were perfect for this.