This stuff? https://www.amazon.com/dp/B08BHVWNWK/ref=cm_sw_r_cp_apa_fabc_GCobGbD7SMRB7?_encoding=UTF8&psc=1
The OD is 4mm but the MP Select Mini v2 wiki page suggests that the tubing needs to be 3.5mm OD in order to fit inside the little threaded tube thing inside the heat sink.
Get this, should work, has a fitting for both end and the Bowden tube will be so much better.