Sebastian Letocart is known for his completion of the Finale of Bruckner’s Ninth Symphony, (he calls himselt Tracotel on Facebook), has a number of other compositions.
One of these is a five-movement a cappella Great Mass in D.
The work sounds like it comes from the Renaissance era to my years, the days of Leonardo da Vinci, with the harmonies and counterpoint somewhat more dense and even more modal.
I can find three of the five movements on YouTube, sung by the Matthew Curtis consort.
The Gloria and Agnus Dei are also available.
I've never been a personal fan of a cappella. My ear wants the overtones of instruments.