Icelandic equivalent of the German surname Röntgen

The latter -gen in Röntgen is definitely a diminutive suffix:

g remains voiced after voiceless consonants in names: Fitger, röntgen. The g in the name Kügelgen is [g], though -GEN ORIGINALLY IS FOR -CHEN; CF. GOETHE's SPELLING: MÄDGEN (MÄDCHEN) (Jethro Bithell, German Pronounciation and phonology)

So the ending -gen in the surname röntgen or roentgen is the Goethian form of the diminutive suffix -chen, related to the Old English suffix -kin. For the Icelandic adaptation we will use -kin as in the following example mentioned in the Íslensk orðsifjabók.
Vilkin k. karlmannsnafn. Tökunafn ættað úr þýsku samanber fornháþýsku Willikin, miðháþýsku Willeke, eiginlegt smækkunar- eða gælumynd af nafninu Wilhelm.

The first element in Röntgen probably derives from the Old High German rant
(shield or 'boss of a shield'), mentioned in the the Icelandic etymological dictionary under the entry 'rönd'), which semantically answers to its Icelandic etymological cognate 'rönd', which still exists in personal names.

So I choose Rönd- as the first element in the Icelandic adaptation of the German name, as in Röndólfur (Randolph, mentioned in the Íslensk orðsifjabók under 'rönd'). Then we add the icelandicized Old German suffix '-kin' and we have our adaptation: RÖNDKIN. As for the gender of the word, we will use the earlier mentioned 'Vilkin' as an example, which is masculine so it requires the genitive -s in compounds: RÖNDKINSGEISLUN, RÖNDKINSSTJARNA