The original cooling system is not well thought out. The air flow is insufficient and is not well directed.
I made this fan duct, after various tests, taking inspiration from various projects (none of which are clearly designed for A20).

This system use the original 40x40 radial fan duct, but pay attention, my A20 is not completely original:

I wanted to adjust the fan speed (small parts, slow parts, surfaces,bridges ecc..), but with the original firmware I only had the ON / OFF option (0 for OFF, 255 for ON, no other value was able to spin the fan).

I then changed the following parameters (I used marlin 1.1.9 bugfix):
in configuration.h:

♯define FAST_PWM_FAN ♯define FAN_SOFT_PWM ♯define SOFT_PWM_SCALE 2 ♯define SOFT_PWM_DITHER

and in configuration_adv.h:

♯define FAN_MIN_PWM 100 ♯define FAN_MAX_PWM 255

I have published a project page for firmware here:
https://www.thingiverse.com/thing:4744713

Eryone ER20

No

Yes

0.12

20%

I used PLA +, I had run out of PETG, but it doesn't seem to have any problems with the temperature.

IMPORTANT: use only supports in contact with the bed (view images for more details)!

For best results use silicone to join the fan and the duct, use washers to not force too much on the molded part