Trouble with that suggestion, Yitzi, is that the D&D rules assume that an insanely powerful elemental is animating a Flesh Golem.
They just don't behave like undead.
For customisation, check out Xenoalchemy
, which has days of different options. I don't think there is a monster customisation thing in there but there are's so many options.
Also, there's the Ashen Necromancer
which has a lot of customisation, though none of it is geared towards actual piecemeal stuff.
Finally, there's a spell, an immensely long spell, in in the ain Ravenloft book called Stradh's Maelific Meld, which has rules for stitching all kinds of things together and animating it. The example is a gargoyle winged, werewolf torsoed drider with the Phase Spider's shift abiliities.