The only thing I've ever seen a couple of the DIY fire pits have problems with was using a metal insert that can't drain. Or a solid concrete base. You end up with a very nice on the ground bird bath and mosquito breeder.
A guy I worked with did one some years ago. His solution to the ash removal and water drainage problem was to take an old drum and cut it to size for the pit. He laid in his rock and brick around, giving it about two inches of space between the actual walls and the drum as well as a couple of gaps in the base for drainage. He built up the base with four bricks that the drum sat on top of and drilled holes in the base. When it came time to clean it out, he just pulled the can out, dumped the ashes and swept the remainder from the pit itself. A drum will last you a few years before rusting out to uselessness. And you still have the bottom half to use when/if it becomes necessary.