Did this for both but it requires making parts aout of a block of aluminum.
The basic idea was conduct the heat to the side where there is
a larger finned heatsink. To do that you need aluminum at least
.187" thick and has a right angle such that a larger finned heatsink
can be mounted with the shortest thermal path.
In my SB1 (nearly the same layout in that corner) I stood the output
transistor vertical on very short leads and used a block of aluminum
with a face of it as the device heatsink and a side face to a larger
fan cooled beatsink from a desktop PC. The block is about
.75x.75.x.75 inch with the .750 x.750 inch face used as the mounting
for the IRF510 and the ,75x.75 side face as the interface to the
heatsink at right angles to the board and off the edge of it.
The heat sink is 3x3" with fins .4" tall and copper. I have done
same for the SBII and the box rear panel (.050 al) is the first
surface and then outside is a finned heatsink 2x4" with .75 tall
fins and its seems effective. The irf510 is mounted using
insulator kit and heat sink grease on all surfaces that transfer
heat. [irf510 to block, block to panel, panel to finned heatsink.]
One last note, even with the existing heatsink a small fan and
vented case with good airflow will greatly improve the
performance of the existing heatsink. I've seen too many that
bottle up heat inside with a pretty and very sealed case.
Also make sure the driver device is not also getting cooked!