You have to change the stack size. I'm pretty sure there's a mechanical code requirement for solid fuel burning appliances have a 6" flue minimum. Only tested and listed appliances can have smaller. The stack needs to outlet well above surrounding roof. For adequate draft, it should be at least 10 feet high and have minimal horizontal offsets.
You cannot load from the top, it must be from the end opposite the stack exit. This is also where combustion air should enter. The air intakes must be controllable. This can be by any simple sliding or rotating plate arrangement that can be moved over the openings at any time needed.
I'm sorry for being blunt here, but you don't know what you're doing. You do not understand in the least anything about the physics of combustion. You stand a very real chance of either burning down your building and/or poisoning yourself and anyone with you with carbon monoxide. There's many little details you must know to install a safe solid fuel burning appliance. Too many to put forth in a forum like this.
If you choose to proceed anyway, at least invest in a carbon monoxide detector and a large fire extinguisher. PLEASE.
Edit 12/3: Nepal! That paints a different picture. While my concerns for your safety remain, your access to resources are severely limited. Here's a few more thoughts on making your installation safer and more usable.
Be sure there is plenty of clearance between components and combustible materials. At least 30" from the box to combustible walls. If there is less space available, place sheet metal heat shields on the walls with an open air space behind.
To minimize creosote build up, if available, use double or triple wall flue pipe. Consider constructing your own from different single wall sizes if necessary. In any case, ensure there is good clearance from flue pipe to combustible construction, especially where it goes through walls or roofs. 2" for triple wall to maybe 6" for single wall. Use sheet metal to fill the gaps. Obtain a proper size chimney brush to clean out the inevitable build up. Use it often.
The flue outlet needs to be well above the roof as I mentioned, specifically, at least 18" above any point of the roof within 10 feet of the outlet. The outlet should be screened to prevent large hot embers from flying out and igniting something in the area.
As Fiasco mentioned, fire brick lining is important. Besides protecting your box, it buffers the heat output to a more constant, comfortable level. Otherwise, your heater will perpetually be either too hot or too cold.
A good draft is your best insurance against carbon monoxide leakage. Do not use your heater unless you've determined that it is inducing a good reliable draft. While a little back puffing when you first open the door to stoke is acceptable, you should not otherwise observe any smoke leaving the box through any opening, ever. Except for the flue of course.