OLD TESTAMENT USAGE: The word "poiema" us used only twice in the New Testament, as you say. But in the Greek translation of the Old Testament, the same word is used several times:
1Sam. 8:8 According to all the works which they have done since the day that I brought them up out of Egypt, even to this day—with which they have forsaken Me and served other gods—so they are doing to you also.
1Sam. 19:4 Thus Jonathan spoke well of David to Saul his father, and said to him, “Let not the king sin against his servant, against David, because he has not sinned against you, and because his works have been very good toward you.
Ezra 9:13 And after all that has come upon us for our evil deeds and for our great guilt, since You our God have punished us less than our iniquities deserve, and have given us such deliverance as this,
Neh. 6:14 My God, remember Tobiah and Sanballat, according to these their works, and the prophetess Noadiah and the rest of the prophets who would have made me afraid.
Psa. 64:9 All men shall fear,
And shall declare the work of God;
For they shall wisely consider His doing.
Psa. 92:4 For You, LORD, have made me glad through Your work;
I will triumph in the works of Your hands.
See also Psa. 143:5, Eccl. 1:14, Eccl. 2:4, Eccl. 2:11, Eccl. 2:17, Eccl. 3:11, Eccl. 3:17, Eccl. 3:22,
As can be seen, "poiema" is not well translated "masterpiece". See esp. Ps 63:9, where it is used synonymously with the word "erga", meaning "works/deeds". Thus, the same way the English word "workmanship" can refer both to a masterpiece and a mundane piece of work, the word "poiema" can refer to both as well.
DEFINITION: This is exactly the way BDAG defines the word: "that which is made, work, creation". Strong's and Thayer's dictionaries agree - as do virtually all English translations of Eph 2:10 and Rom 1:20.
NEW TESTAMENT USAGE This understanding of the word must also be applied to the contexts in which this word is used in the New Testament: Rom 1:20 explains how we know God's character from the "things he has made". Eph 2:10, about us being God's "poiema", comes after several verses in the beginning of ch2 talking about our sin nature. And though in these contexts the word "masterpiece" could replace "workmanship" without detracting from the meaning, as I've argued, that's simply not what the word means.
CONCLUSION: Neither the definition of the word ποιημα, nor its LXX usage, nor even its NT usage, calls for understanding or translating it as "masterpiece".