The first and most basic thing that you can try doing, is to use your hands as an apparatus to help increase the amount of sound that you produce. All you need to do is hold your hands across your mouth before you speak almost in a cup like shape.
This will help boost the amount of sound you produce. However, this method is not very effective and will have only a small impact on the amount of sound you produce.
Using a magazine if it is close by can prove to be far more effective as compared to your hands. Now if you go on and make a vague cylindrical shape with the magazine. Now make sure that the end that is going in near your mouth is thinner than the other end. This will amplify the amount of sound that you produce and will make it sound a lot more like a mega phone.
However, there is only so much that the magazine can do, since it is not really designed to do this.
Another far more effective way to go about this, is to per-prepare a makeshift megaphone for yourself. In order to do this, you will need a chart paper and some method to hold it together.
Now the chart paper needs to be of good quality in order to get the best possible effect. Roll up the chart paper and shape it up like a mega phone. Now use tape or something else to hold it in place. In order to prevent it from wearing away, apply tape to the end where you will be speaking from as well. This will prevent split from ruining the paper.
Now use this whenever you need to, in order to get the best possible megaphone effect, without using a mega phone.