So instead I made two additional layers of card, both with the same hole cut into them and glued these to the hull. For the engine parts- or at least visible bits of the engines workings I mostly used more 40K bits, but I also used a cable grip from the inside of a socket for a fan and some iPod earphones for cable. The large piece on the top right is supposed to be a heatsink or something similar, and is the foot from a Tau vehicle landing leg, left over from the radar dish on the turret.
I felt the hull top looked too bare just with the engine in the middle so I used yet more 40K bits- aerials and the barrels from a Tau Burst cannon as exhaust pipes.
Now I think the build stage is more or less finished, though I may sand down the milliput as it isn't the smoothest of jobs- lots of lumps. I've also decided on a name for the tank, which I think suits its anti-infantry main gun- the Rasenmaeher, or lawnmower. Anyway, I'll leave you for now with what the Rasenmaeher looks like at the moment. Painting should start on Thursday, after I've borrowed a friends spraygun.