I like the concept of the Cap multiplier. The ideal choice for the
NPN is a Diodes Inc (nee Zetex) ultralow Rce(sat) device, which has inherently high hFE so that when the device saturates instead of a gain of 10 like normal transistors, you get a gain 4x to 5x more and 25% of the Vce drop or Rce (ESR)
**broken link removed**
This tiny NPN device ZXTN2010GTA for $0.90 at Digikey
has Vce (sat.) = 260mV @ 300mA, 6A BVCEO = 60V : RSAT = 35m ; IC = 6A ; 3W max (good for a 15W supply)
and hFE (min) = 100 @ 2A, 1V
Rce(sat) = 35 mΩ @6A for a Low Equivalent On-Resistance
Consider a bigger part for more power.