Gas compression oven

Hi everybody, excuse my English…

excuse if this post is in two different section of the forum, but I did not figure how to delete it…

For some important economic reasons, I want my compression machine to use a gas burning solution, I first looked for a gas oven but I did not find any cheap one who works…
So I m now about to start building my own, I have two designs, one using refractory bricks and one using isolation fibre. As the melting temps for thermo plastics is around ± 200 C° I was hoping that the solution with isolation fibre will be the best (as it is from far the cheapest and the lightest(less metal, no bricks)
What would you advise me? What do you think of the designs?
By advance thank you for answering.