Low Resistance Logic Level MOSFET
To avoid using a heatsink the FET needs to dissipate less than 1.25W (assuming TO220 case with Rtja=40ºc/W and 75ºC junction temperature at 25ºC ambient). Watts = I2R, so at 20A RDSON needs to be less than 1.25/202 = 3.125m&ohm;.

So look for a logic level FET rated for 30V Drain to Source and RDSON = 3m&ohm; or less at 75ºC. Many suppliers websites have a parametric search function that you can use to narrow the field down.