A root in Quenya Notes (QN) from 1957 glossed “solid, tangible, firm” with derivative Q. ronda “solid, firm” (PE17/183). It had a couple variants: √GRON > S. grond “solid, tangible, firm”, also the name of Melkor’s hammer, as well as √SRON > Q. hrón/S. rhû “matter” and Q. hrondo/S. rhond “body”. The root √SRON appeared again with the gloss “flesh, substance, matter” in etymological notes associated with the essay Of Death and the Severance of Fëa and Hrondo from the late 1950s, along with both Q. hrón “matter” and Q. hrondo “physical body” (MR/231 note #26). However, in the title of this essay hrondo was revised to Q. hröa (MR/217), elsewhere derived from primitive ✶srawā (MR/350); see the entry on √SRAW for further discussion (MR/330).
Regarding S. grond: in The Etymologies of the 1930s this word was derived instead from the unglossed root ᴹ√RUD, which had derivatives ᴹQ. runda “rough piece of wood” and ON. runda > N. grond “club” (Ety/RUD); it also had strengthened variant ᴹ√G-RUD (EtyAC/G-RUD).
Neo-Eldarin: Despite the change of Q. hrondo >> Q. hröa, I think √SRON might be retained in the more limited sense “matter” rather than “flesh”, along with related root √(G)RON “solid” from 1957. As for ᴹ√RUD, it might also be retained, with S. grond “solid; club” being a blending of ᴹ√RUD and √(G)RON.
This root first appeared as ᴹ√ROD “roof, cave” in The Etymologies of the 1930s with derivatives like N. rosta- “to hollow out, excavate” and ᴹQ. rondo/N. rhonn “cave”, having instead the sense “domed roof” in Ilk. rond which served as the basis for (at the time Ilkorin) name Ilk. Elrond = “vault of heaven” (Ety/ROD; EtyAC/ROD). In the Quendi and Eldar essay from 1959-60, Tolkien instead had the second element of this name as S. rond “a vaulted or arched roof as seen from below (and usually not visible from outside); a (large) hall or chamber so roofed” as a derivative of ✱rono “arch over, roof in” (WJ/414).
Neo-Eldarin: For purposes of Neo-Eldarin, I think it is best to assume this root was √ROD, because some of its early forms (rosta-) can be derived from this root but not later RON(D)-.