Rogatchover on building the teivah: is the mitzvah the maaseh or the totza'ah?

The Rogatchover asks what the geder of Hashem's mitzvah to make a teivah was.  Was it the עצם המעשׂה, the act of building the teivah, or was it the end result, the תוצאה, of having a teivah, which was crucial?

I would say mi'sevara that it depends on what the real reason for building the teivah was.  If the point was to have a place of refuge for Noach and family during the flood, then all that matters is the end result of having a safe haven.  If the point was to raise awareness of the impending flood and thereby arouse the people to do teshuvah, then process of building, which people would take note of, was crucial.  (Baruch she'kivanti, my son pointed out that the L. Rebbe makes this point in a sicha (vol 15).)

The Rogatchover in his unmatched genius brought a proof from the following gemara (Sukkah 52b):

״וַיַּרְאֵנִי ה׳ אַרְבָּעָה חָרָשִׁים״, מַאן נִינְהוּ אַרְבָּעָה חָרָשִׁים? אָמַר רַב חָנָא בַּר בִּיזְנָא אָמַר רַבִּי שִׁמְעוֹן חֲסִידָא: מָשִׁיחַ בֶּן דָּוִד, וּמָשִׁיחַ בֶּן יוֹסֵף, וְאֵלִיָּהוּ, וְכֹהֵן צֶדֶק.

Why are these 4 people called craftsmen?  Rashi explains that Mashiach ben David and ben Yosef will build Beis haMikdash; Eliyahu built a bamah on Mt Carmel; Kohen Tzedek is Shem ben Noach who helped his father build the teivah.

Had Noach been commanded in the עצם המעשׂה of building the teivah, then how could he have allowed his son to help?  He was given the mitzvah, not his son, and there is no din of shlichus for a ben Noach!  It must be, says the Rogatchover, that all that matters was the תוצאה, but who did the building or how it was done was not important.