use wood/concrete/metal beam [whatever fits in with the structure]at the correct height for the door frame strong enough to support the door and whatver infill material you have above
theres a fair chance 3x2" timber will be more than adequate
mine has a timber beam which forms the "head" of the doorframe fixed to the walls, and the gap between this and the structural opening is infilled with ply, stained to match the frame and door. I have added insulation to the inside of the infill which in my case is only a couple of inches.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local