High current DC motor driver mosfet
You can do this with no heatsinks .Keep the PWM freq as low as    acceptable  audible   motor noise will permit .Motors are different but but you should be at 20KHz or less .Low frequency operation of low voltage mosfets will mean that the switching losses are well below the conduction losses and for your ballpark calculation can be neglected .I am assuming that your battery voltage is low .Your P chan fets allow simple circuit but they do have more on resistance .The reason for this is electron mobility Vs hole mobility .It may be better to use an all N chan H bridge and a high side driver .Say if you google some 10milliohm devices that are in say a D squared package which are easy to hand solder .Mosfets share well when in parallel so think about 2 in parallel which means 8 devices .Now each device could get 10 amp and would waste 1 watt . 1 watt on a D squared pack on a well designed PCB with 2 oz copper or better with lots of Vias will be fine for operation at normal ambient temps .You could employ a hall effect device to allow the power wasting current      sense resistors to be removed .