Which material should I use to print a mounting adapter for the y-carriage?
Any thermoplastic is a bad choice to carry a heated bed. For an unheated bed, monolithic (cast, not printed) acrylic plate of sufficient thickness can be used.
To get the needed stiffness and temperature resistance, steel plate of about 2+mm is the best choice, followed by aluminium of a higher thickness. The latter can be sawed with woodworking equipment, if done slowly.