Most of us build plastic kits but all of us deal with the same issue; the plastic walls are way to thick in most cases. There is a perfectly good reason for this and it is due to the mold making process.
There are times when we need to create holes in the plastic, for me it is usually because of rust damage or a wreck. Just poking a hole in the plastic will look grossly unrealistic. The easiest way around this is to thin the wall and then you can add your holes and cuts to your model. Most of the time I like to do this before the parts are assembled for obvious reasons. In some cases, like this project, I decided to add rust damage after I built the 55 gallon drum kit.
Start by thinning the wall of the plastic part. You really only need to thin the area that the hole is going to be. I like to sand it down until I can see light coming through the area (If it gets very thin in the area, the plastic can warp forming a kind of bubble but that's OK).
Once you have the desired thickness, using a hobby knife or other sharp object, you can poke and cut into the thinned plastic.
It is a very easy way to add some additional realism to your plastic models.