Hi Heather, I have sorted the roof by soldering 4 small lengths of brass tube, 1.55mm OD and .8mm ID tight into the 4 corners of the cab. So that they are not seen through the front and rear cab windows, I have also filed the tube flat on two sides and countersunk the top of the tube before soldering. I then soldered 4 'L' shaped lengths of .7mm brass wire at 45 deg nearly into the four corners of the underside of the roof and sharpened the ends of the wire. If you get your measurements just about right the wire can be slid into the ends of the tube and friction should hold the roof down. You can also put a slight bend in the dropper wire to help with this if needed.
I have packed the loco up to take to Kettering in the morning but, if you need photos, let me know and I'll take and post some later in the weekend.
Kind regards
Sandy