Advice needed: how to level an archway under stairs

[polygon.io]

Hi community
Does anyone know how you can level a ceiling render which has a slight slope so that a door can fit nicely.

I've attached a photo for reference.

Research suggests using hard foam board. Scrubbing the slope... Cutting and gluing it on.

Not sure if anyone here has better advice

 

[polygon.io]

 

[big-all]

thats going to be a messy simple with the right tools difficult with ought
start with something like 2x1 par[21x44mm] dig out above the doors to expose the stair timbers trim the 2x1 at the correct angle to saddle [sit over ] the timbers flush at the front and 4mm gap to the door tops

 

[polygon.io]

Hey @big-all I forgot to to mention that the stairs in our house are concrete so I think it may be easier to build a square frame from laminate then for the top I may just use some sort of foam board .. scribe cut ... Shape and glue on then usual jointer compound to finish. I'll try this approach and update on progress

 

[big-all]

going to be really difficult as you only have a say 15-20mm transition area thats a triangle
perhaps an"L" shaped trim no nails in position
or a bit off shaped timber with reduced size doors to give enough room for a reshaped bit off timber to fit in