The affix yat comes in the sense of 'made' after the word chandas being in the Instrumental -3rd case in construction.,
Thus छन्दसा निर्मितं = छन्दस्यः 'made at will.' The word छन्दस् is here synonymous with इच्छा 'will, wish, desire, fancy' &c, and does not mean 'metre or Veda.',