Welcome to the group.
Most people modify the airbox. The main idea is to let in more air, so if you can do that while keeping out the dirt (panty hose or what sounds even better - the filtered air vents, Uni brand I think), then that is the way to go.
Even if you do NOTHING to try to get it to make more power, I recommend that you get the bike properly jetted. I seriously doubt it is jetted properly right now with the factory jetting, so you may want to look into that first, or right after doing the airbox lid modification. Getting it jetted propery is just proper maintenance. If it gives it more power (it will), then so be it.
But seriously, learning how to properly jet the bike is an important skill. You should learn that even if you are not interested in more power. Changes in altitude, humiity level and temperature can all affect the jetting, and I suggest you learn how to jet to deal with those changes. Too lean = engine damage and too rich = build up of carbon inside the engine, especially on the power valves, which if not properly maintained, can lead to engine damage from them being gunked up (sorry to get technical).
If you are looking for inexpensive, then jetting is your answer. If you get a pipe, you will probably need to change the jetting. If you get your carb modified, you will probably need to change the jetting. You may as well ago ahead and learn this important maintenance technique now.