Homemade Dome-Pizza-Oven issues

I am a rookie DIY dome-pizza-oven builder… trying to make a really cool, long-term (hopefully), pizza oven in my back yard and building a deck around this since i have the time before I am called back to work… I have the base built, and a Yoga-Ball firmly in place within a 1/2" piece of plywood, so this will provide a mold for a refractory to build… I bought Lincoln 60 Fireclay, (mixed it to what I read was a correct ratio of 27% water to the product), and applied the clay to the mold. It cracked as it slowly dried–badly. So bad that I ‘scrapped it’, and want to start over with the last two bags I purposely ordered in case this happened.

My question is this… instead of using -only- the Fireclay, I am searching for additional mix to prevent the disasterous cracking, and considering adding some Silica Sand, and Portland cement to this next attempt. Does this sound like a decent plan? Once this is all dry, I was going to do a layer of Perlite and portland cement mix, letting that dry, …following a more cosmetic appearance of old brick pieces & a ‘pseudo’ Italian/Tuscan looking plaster.

(As far as the oven’s surface, I have applied that already) a layer of perlite and portland concrete successfully, however the organic fireclay I put down after…yeah, it cracked like the Sahara Desert… thinking of making my own firebricks instead as the oven’s surface…
Is this ok?!