A fire in combination with a reflector in front or even inside your shelter is a really good way to get a lot of warmth.
As for the type of fire: It really depends on your type of shelter, but the so called top-down fire is the one I can really recommend. If done right, this will warm you while building it :) and once lit, will provide you with around 6-8 hrs of warm fire without the need to care a lot about the fire and without too much smoke.
The key concept about this fire is to use thick wood (at around 15-20 cm in diameter) for the base. Use 4-6 of these and lay them down parallel to each other. Upon this base, put a little sand/dirt. Just enough to cover the space between the logs. Put the next layer on top in a 90 deg angle, while using slightly smaller wood. Repeat this, until you have 5-7 layers.
On top of this yenga-pile, create a normal fire and feed it until the first layer of thicker wood burns as well.
The time this fire burns is determined by a few factors: Type, thickness and humidity of the wood used. And obviously on the number of layers, amongst other factors.
Now what's so special and cool about this?
- There is no or just little wood above the flames. This reduces smoke a lot. Also the construction allows from loads of fresh air flowing in from the bottom of the flame.
The fire burns from thin to thick, top to bottom.
Less maintenance. Sure, this takes some time to build and collect the wood for it. However, you don't need to take care of it during the night very much. Means more sleep for you as well as longer sleep periods.
Hot ashes and coals will not be wasted, but start to burn/dry the wood under the actual flame. This way you can even use quite wet wood for the base.
Safe: This fire is relatively save. This means: Most of the time the burning ashes and flame won't touch the ground and heat it up.
A variation of this, suitable for an a-frame shelter would be a long-log fire. However without the correct tools (aka a good and long saw and dead trees with a diameter of at least 40cm) this is an impractical fire and takes quite some time to build.