You can buy your own small lantern that will fit in a tent cheaply enough, but where's the fun in that? If you want to make your own, customised to whatever specs you want, Adafruit has a guide.
By the end, you'll have a lantern that isn't too bright, but illuminates a tent enough that you can do whatever it is you need to do. Other than some soldering skills, you'll only need a handful of parts, including a battery, some LEDs and a case. If lamps on the market aren't good enough for you, this is one way to make one for yourself that will behave exactly as you want.
Tent Lantern [Adafruit]