The stock Tekno SCT410 Servo horn is a bit different than normal horns, as it has some height to it, to mount the steering linkage under it. Here's how I fixed the issue to incorporate an aluminum servo horn from OFNA. Below are the model numbers available at Nitrohouse:
OFNA 23T:
OFNA 24T:
OFNA 25T:
I did not verify these, but these Serpent versions are said to work:
Serpent 23T: SER600463
Serpent 24T: SER600464
Serpent 25T: SER600465