For smaller craters, use the metal lids off curry or jam jars glued onto thin MDF etc. Use DIY filler, or DAS etc to build up the rim of the crater and then use PVA glue and sand to texture it. A quick splash of brownish-grey paint and job done!
It works very well - I had to help my son do a model moonscape for his school project and his teacher was very impressed

You could also use a strip of card or plastic formed into a ring and taped or glued to the base as a "skeleton" for larger craters.
BTW, I got this idea from a group of WW1 gamers at a local show in Birmingham several years ago and their terrain looked superb using this method.