The 13" UTG Pro AR15 Free Float Quad Rail might work for that build since the manufacturer states its exact length is 12.6". However, the manufacturer states that it is compatible with low profile gas blocks but doesn't mention A2 FBS. So to be safe I recommend using the handgaurd listed below.
https://www.opticsplanet.com/leapers-utg-model-4-15-mid-rifle-length-quad-rail-system-black-mtu007.html