I'm planning to use a homebrew cast over a gymball. No needles or fibres.

You can see the plan in the attached PDF.
I live in sweden, so it can be kind of cold nights still, could very well turn to below 0 C during the nights and early morning. Can i start casting in my garage?, not heated. The long term weather cast says above 0 after this weekend.

i will try to the the cast of homebrew and the bottom plate of concrete in the garage, after that im afraid i have to move outside and continue the work where the final placement will be.
I'm gonna use a wooden furniture as a base and my plan was to have it on my wooden deck, but i realized it might be to heavy for that, so i maybe have to put it outside of the wooden deck.

When i put my pieces together and apply my insulation layer(s) should i put a plastic coverover it to protect it from rain? im not sure i will have roof over it.
The clay i found is for tiled stoves, will that be good enough? cant find "fire clay" here at a decent price, refractory mortar is easy to find, but its not what i want?

Please look at my drawing and point out possible misstakes i might make.

the calculated weight is over 300kg, are my numbers close to reality?

Attached Files