What do I use as a door sill

Hi all. I'm a new user to the group, reasonably competent at DIY, but this one's got me stumped and I need your help.
We had a window taken out of an old bedroom and the opening extended to fit in an aluminium sliding door as a new entry to our backyard. The house is brick and the subsequent hole was cut out of the brickwork. The door was fitted and works beautifully, and we'll eventually build 2 steps down from the door to a new deck, but directly under the outside door frame we have a three inch gap, to the top of the cut brickwork which hasn't been finished by the door fitters. They suggested to fill the hole with bricks, but it's too short for a full brick height. We can still see the flashing at the back of the opening under the door frame but:

(a) what can I fill the hole under the door with, and
(b) what can I use for a sill/top step on the outside of the door opening?

Tim


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