Casting my first oven. Help

[sonomacast]

I am in the process of casting the dome of my oven i am in the UK and see that there are various products available i guess castable 1400 (Refactory cement) is what i need but does it need reinforcement such as fibre added to reduce cracking. also i have read on the net that it is not such a good idea to cast in one so should i cut the dome into 2 or 4 segments after it has cured then cement back together. (I guess this will work as an expansion joint) Any ideas on this project will be greatly appreciated. Thankyou