Mustrum_Ridcully
Legend
The average monster is just a bunch of stats. The specific fluff gives it a distinct feel and look. So, it's entirely possible to just grab a bunch of random monsters, and change the look and fluff to what theme you have in mind. A crystall spider, for example, could start as some vrock. The vrock's dance would be changed in description to some crystal web vibration effect, special attacks get a crystal/poison/web look, and so on.
That's not ignoring the system, that's using the system.
Assuming that you find monsters in the level range in question that are suited for reflavoring. I don't know exactly anymore, but I think Vrocks for example would be too high level.
And the finding thing is yet another problem. It works fine if you remember a monster that could work, but if you are skimming through your MMs just to find a monster that might have abilities that can be reflavored, you still spend a lot of time. Especially considering that its not enough to just skim the flavor text - you have to read through the stat block and see if it suits your need.
I just don't work like that. I haven't memorized the contents of the MM (I might know what a monster can do if I see it, but I don't have a mental of index "monster with breath weapons" or "monsters with effects that could be crystalline or web flavored").