As Thule Spring is thought for kids from 6 months old, there is no Thule bassinet/carrycot for this model. Do you think some kind of a universal carrycot could be used for a newborn? Thank you!
The stroller is not meant to be used with a carrycot, you are right. What can be done, though, is to use a reclinable car seat instead, for ex. a Joie i-Level or the Kiddy Evoluna line, with the Maxi-Cosi adaptors it will work.
Apart from that, the Cot To Go from GB could be used, I believe, with the same adapters, however I cannot say how the center of gravity changes, and it could look funny over the song. I cannot recommend this way, but it should be possible.
Lastly, you could get a soft, insert cot / coccoon, for ex. from Hauck, britax or Valco Baby (just ask around in any larger baby shop, they will probably have some), and insert it in the reclined seat - it should create a cozy enough environment for a newborn.