Open the Console with the "~" (tilde) key

Turn 180

bind STEAMVR_SECONDARY_a_turn180

Smooth Turning

bind STEAMVR_PRIMARY_AXIS2_lookLeft bind STEAMVR_PRIMARY_AXIS3_lookRight

Alternatively, just chain these together with a semi-colon

bind STEAMVR_SECONDARY_a_turn180; bind STEAMVR_PRIMARY_AXIS2_lookLeft; bind STEAMVR_PRIMARY_AXIS3_lookRight

Go into SteamVR Controller Bindings

Map as you please :)