Go for 4043 which is most commonly used for aluminium 6xxx series and available with ESAB and LINCOLN.
The heat input shall be restricted to as minimum as possible as the tensile strength reduces significantly during welding of this material. Also hardness also reduces significantly in HAZ area.
Heat treatment is recommended after welding in order to restore the mechanical properties of 6061-T6. Heating at 530-550 deg C and following quenching in water and then reheating upto 150 deg C would do the job.
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