Here is a basic circular firepit mould.

Design is for 25 bricks per layer, each brick equates to 75mm high, so a standard 750mm height firepit will need 10 levels or 250 bricks to be cast.

Suggest you use fireproof cement to cast these, as regular cement is porous and will trap water in rainfall, when heated this can then expand, causing the bricks to shatter.

There is a left and right mould, these are designed to be placed side by side, joined with the dowels, and then clamped together with a simple clamp or strap, you then pour your mix into the top through the hole until level with the hole, and wait for it to be set.

Once set, remove the clamp and you should be able to tap the mould and have it come apart and you can re-use the mould.

Alternatively, you could use regular cement and mortar and make this into a kick arse wishing well for your garden.