R Abraham S. Abraham discusses a related element in the volume 1 of his Nishmat Avraham pp. 166ff. - a 3-volume sefer of medical halacha.
He explains (based on Shulchan Aruch HaRav) that the assembling of items on Shabbbat is only prohibited if it will definitely not be dismantled on Shabbat (i.e., building a permanent structure). Therefore, according to him, there would not even be a violation of Rabbinic prohibition in assembling something that will be dismantled after use.
He applies it to syringes and needles but it feels it could be applied to a prosthesis as well.
I also saw that R Moshe D. Tendler and Fred Rosner write in their article The physically and mentally disabled - Insights based on the teachings of Rav Moshe Feinstein
Other prostheses, such as artificial limbs, may also be put on, taken
off, and worn - even through a public thoroughfare - because these
devices are considered part of the person's body.
Both Nishmat Avraham and Shmirat Shabbat KeHilchata (18:15) allow going out and wearing a prosthesis on Shabbat (where there is no eruv).
See also this related article from Ohr (credits to Kazi bácsi for locating it).
Of course CYLOR before applying in practice.