This is my success project of fittings for aluminium-plastic pipes (20mm). It is suitable for non-pressurised pipes. You can print the header and the fitting separately to ensure the best precision. The top of the header must be smooth - the rubber seal is inserted here and pressed in with the cap part.
The T-part must be glued with the headers. You can print the T-part separately or in the version with the 2 headers included in one print.
The only needed extra part is the rubber seal.
I printed it in PETG with 0.8mm nozzle and 100% infill. Im using the prusa slicers variable layer height - headers are printed with 0.2mm layer height and the last of body with 0.55mm layer height.
There is no need for supports - just in case you will print the T-part in standing position.

UPDATE:
+Added 3/4" to alpex 20mm adapter

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