How to fill 1" gap between door frame and lintel

[Greggerly]

hi guys
I will be cutting opening in the garage and fitting personnel doors. It seems I will end up with a 1" gap between the door frame and lintel. Whats the best way to fill it? Simply fill it with mortar or is there a smarter way? I know there are infill panels available but it seems overkill as they're mostly for much larger gaps.
thanks

 

[He who knows]

Timber and a wider architrave?

A 1" thick piece of clear or translucent acrylic, projecting a little over the door on the outside, with an LED strip fixed to the inside edge?