Integrate DC/DC converters in a friendly package

Non-isolated? A tiny synchronous switcher chip and an inductor and a couple of caps will do it. TI has an online form you can fill out that makes recommendations.

LMZ21701 from TI gives up to 6W (6V at 1A) and its package is microscopic. Unfortunately isn't simple to mount on the PCB (the pad is behind the package).

Pin*efficiency = Pout

with efficiencies in the 90's the dissipation is a few 100 mW

supposedly good for several amps, and same size as a TO220

-Lasse

So, you've painted yourself into a corner. We've all done it. Next design, try to leave more space, or start with a more flexible power supply section.

This time, consider options other than 'small so it fits': mezzanine board, reroute other circuitry, increase size of PCB, redesign to utilize existing power, add auxiliary outputs to main power, encapsulated DC/DC module (basically, this just means getting someone else's design of mezzanine board).

Have you looked at the RECOM range?

Maybe really small, but easily-soldered parts? Converters in SOT-23 packages, running at high frequencies like 1.4MHz. E.g., the NCP1529, TPS562200, ADP2301, SY8008C, AL8805, LMR10510, and XC9257. These go above 1A, but there are others that use an external MOSFET to higher.

I often resort to mounting smt parts on the bottom when space is scarce.

you might want to replace the pot with a couple of resistors to lock in the voltage..

A bit pricy compared to this for $1.90

The TI Simple Switcher series has chips with a small number of pins.