On my SB1 20M whch had a very similar outline and layout just different display.
I used a block of aluminum behind the final to couple it to the aluminum front panel
in place of a spacer and an Lshaped aluminum (1" Angle alumnum) to go from
behind the irf510 to the block on the other side of the board. The Angle aluminum
was about .1 thick. The idea was conduct the heat to the front panel (.080 aluminum)
and take advantage of the surface area. For voice it never even got warm.
Sorry no pictures.
For digital work the heat sink must be huge compared to the supplied one as the
full power transmit time is 100% effectively. Voice work the average power is
under 20% often less. Also the box must have air flow to allow heat from the final
and its larger heatsink to escape the case. Consider using a small 40 to 60mm
fan for cooling as well. The major trick is do not make the IRF510 leads longer
or stability may be compromised. also note the tab is the drain connection so
it has the main power on it and RF so it must be insulated from any cooling
structure (it must conduct heat) and that structure must be grounded.
The IRF510 can stand up and have a block of aluminum to couple the
heat to the back of the case. For that I'd say a block .625"W x1"D x1"H
might do well to get to the edge of the board and couple heat to a larger
finned heatsink. the IRF510 is mounted to the .625" wise face and the
1"x1" face to the heatsink. Again insulating kit and ground the heatsink.