You may only do it a few times so why waste money? DIY this and the lighting is a lot better because
it's diffused.With ring flash unless you add difuser it look hard/harsh..I'm sure you want to do beauty shots of your insect friends?
Sorry for the late reply...unfortunately if you want TTL flash capability then you are limited to use Sigma dedicated flashes only like the older smallest EF140s,EF610 or newer EF630 (not cheap)Make sure it's for sigma cameras as they make for other brands.
Cheaper manual flash Yongnuo 560 IV or similar china clones.