Choosing a power transistor with low dissipation

A linear regulator is a lossy delivery system especially when coping with high load currents. No matter what output transistor you choose, the same heat power will be generated. So, if you stick to linear operation, the only option is to provide a better heatsink to remove the heat power more effectively and thus lower the temperature of the transistor.

Or, use a switched mode controller.

+1 on what Andy said.

Not what you asked, but what you need is a bog standard low side N-FET PWM driver.

^{simulate this circuit – Schematic created using CircuitLab}

Such a device is most likley available on eBay as is. An adjustable 24 V, 8 A capable buck converter would also solve your problem. eBay link