Making complex structure from cement board

That sounds like the sort of problem they would set us in Technical Drawing when I was studying engineering.

Have you considered using aluminium, rather than steel? Easier to bend and work into shape and you can pop rivet it into place.

Alternately, use copper, which is even easier to shape, and make a decorative hammered copper hood.

thin sheet metal you could bend to any shape, whack over the edges easily etc.

So many possibles

Use copper. Expensive but will look gorgeous Use fireproof board and then cover in something more attractive Use (small) bricks e.g.

Use a hood bent from sheet metal, with u-channel riveted to the edges and fire-proof board slotted into the u-channel, then metal trim pieces at the board joins, screwed through into the metal hood?

How about a ‘chicken wire’ support coated in a mix of either perlite / cement or vermiculite / cement. The light weight concrete is heat resistant, you can skim it to get a neat finish, and probably get a complex shape. Possible run a length of light weigh flue pipe through it and some metal struts for support. Even use thick cardboard underneath while shaping it. It will burn away later.

If the sides were straight and the front sloped then you could use angle iron but with all faces sloping up similar to a hipped roof the angle will be wrong.

Richard