Elevated Hearths

A question about elevated hearths. I have just bought a Harvia Legend 240 which will sit on a tanked wood framed limestone tile floor. There is a drainage channel running across the floor that has a slight angle into it. I will create a rear heat sheild using cement board and corrugatted metal.

I want to make an elevated hearth for it to sit on. I have extra cement board and tiles left over or could make something using metal and fire board.

The Harvia Protective Bedding (WL100) is around £300 and I am relucantant to spend so much on it.

What do you think I could use/build?