Flashing door sill

[idris]

I'm building a SIPs garden office. ( Isn't very *****!?! )
The edges of the SIPS floor panels are wrapped with DPC, and there's about 80mm overlapping on top of the floor.

The exterior will be wrapped in breather membrane, including into the door reveal and down to the bottom of the floor panels.

There will be a UPVC door (and sill) 2m wide.

 

My concern is how to waterproof the bottom corners of the door aperture.

 

I've seen multiple recommendations to apply Tyvek FlexWrap NF over the the edge of the floor and up the reveal.
That's fine, except I don't really want to shell out £70 for 23m when I don't need as much as 3m.

 

My other solution is to use some extra DPC and a load of bituminous or polymer mastic at the corners.

 

Any other genius suggestion?

[Temp]

I would be tempted to make the door reveal several inches wider than the door sill and run the breather and cladding down the reveal to below the sill ( same level as the rest of the building). Leave a 1" gap at each end of the sill so  it doesn't have to be sealed against the reveal.