From your original post I was not absolutely sure if you were having difficulty keeping the temperature down to 225 or whether you were having a problem getting it up to 225 ?
Some general questions?
Which is the main problem? Getting it up to temperature or stopping it from getting it too hot?
Is the problem also maintaining the temperature over time without the coals burning out too fast?
As Steve mentioned, for temperature control you will need a tight fitting lower door with adjustable air vents.
If the flue is directly above the cooking chamber then you may need to introduce a baffle just below to stop too much heat loss through the chimney but without restricting air flow. Some tin foil or a steel sheet placed on the very top cooking rack may do the job.
Looking at the photo it is not clear if the fire pan has any holes in the sides/bottom to allow the air to get to the fire. If it is a solid pan then this could affect the efficiency of the burn. You would be better off having the fire rack rested on a couple of bricks rather than the bowl as this will ensure a more even air flow through the fire and also to allow ash to drop below. The air vent in the new door should also be placed at the bottom, roughly in line with the fire rack. Some also put a smaller vent towards the top of the fire box too to help fine tune air flow.
What fuel are you using? For such a large brick over it will take quite a while to get the cooking chamber initially up to temperature - but then it should give you consistent heat over a period of time. Getting it up to a stable cooking temperature could take an hour or more.
You will need a fuel that will provide a high heat output over a sustained period of time. You can do this with wood splits but it will burn a lot of them initially. You may want to augment these with some good quality briquettes or lump charcoal as these will give a higher heat output.
Things to try...
- Get the door in place with a low air vent
- Open up underneath the fire rack to ensure an even air flow
- Add a top baffle to help retain heat in the cooking chamber
- Use a fuel with a higher thermal output than just wood
