Help with suffixed singular forms for stem-changing nouns

I have never understood how to add case and possessive suffixes to singulars of nouns with stem-final consonant clusters. Some examples:

Sindel "a Sinda" (pl. Sindeldi)
*Etyangol "an Exiled Noldo" (only pl. Etyangoldi attested)
oromar "a high hall" (pl. oromardi)
hen "an eye" (dual hendu)
oron "a mountain" (pl. oronti)

When a consonant-initial suffix is added, what happens? Is the etymological vowel restored ( Sindeldan, Etyangoldon, oromardan, ?henden, ?oronton)? Is a filler vowel inserted (Sindelden, Etyangolden, oromarden, henden, oronten)? Does it depend on the word?