Major spoilers about the nature of Reapers in Mass Effect below.
It's said that Reapers are made of and in the image of a specific species that's been harvested. They're supposed to be "nations", collections of specific species to preserve them, hence the Human Reaper at the end of Mass Effect 2. Harbinger was presumably made of some Insectoid species.
However, Reaper Destroyers all look the same. They mostly look like tiny versions of Harbinger with less flourish. It makes sense that Reapers need simple, small attack units, so I suppose it makes sense that Destroyers could be made out of any species.
For reference, this is a Reaper Destroyer as presented in Mass Effect 3:
Is there any evidence throughout the series that suggests how Destroyers are built? They seem contrary to the normal construction of Reapers like the Human Reaper and the failed attempt at a Prothean Reaper.
Best Answer
Why are they similar?
Externally, reapers follow a set design (though the armada seen at the end of ME2 shows that there is room for some variation), but internally capital ships contain a core which is roughly in the image of the one species which is harvested to create capital ships.[1][2]
What are destroyers made of?
Whilst it is not definitely specified (as far as I can find) whether destroyers contain such a core (it could be inferred from [2] but that may just refer to the capital ships), the codex says that they are created from all the other species which are sufficiently advanced to be harvested (Remember that not all species are culled in the cycle - Javik references this many times in ME3) but not the chosen 'capital' race.[3]
References:
[1]Codex: The Reapers - http://masseffect.wikia.com/wiki/Codex/The_Reapers#Reaper_Variants
[2]Interview with writer, Mac Walters - http://www.gameinformer.com/b/news/archive/2011/04/21/mass-effect-a-q-amp-a-for-hardcore-fans.aspx
[3]Codex: The Reapers - Same link as [1]