Octagonal window fitted-sill advice

[ssray]

As per title I fitted a octagonal window yesterday and now I am stumped as to what to do internally.
Each edge is just under 29(11.5inch)mm and the window is set back just over 15mm(6inch)

If I get some were local to cut the wood/upvc for me what angle would I ask for?

Cheers
Ray

 

[big-all]

carry the plastered reveal all the way round

 

[ssray]

Thats the problem...my plastering is poo, hence using plastic to cover it up.

I have some soffit that should do, its just cutting 8 29cm bits with the right(or wrong) angles now
Ray

 

[big-all]

could you not make an 12 mm mdf disc with a hole in the middle with a 3mm rebate for flexi ply or hardboard to line the reveal

have it about 3inches wide around the opening place it on the face off the wall

 

[ladylola]

Angle would be 67.5 degrees.
Possibly the best way would be to make up a mitre box to those angles. The actual angle you need is the angle between two adjoioning window sections equally divided. Practise on some mdf or similiar until you get it right and start on your plastic. Plastic can be cleaned up with a sharp block plane.

 

[ssray]

Thanks ladylola, I will try that
cheers
Ray