RJ's heat syncs are superior, but it is possible to make one yourself.
you do need at least an "L" shaped reasonably heavy aluminium piece 7" wide with about 2.5"-3" tall and the bottom of the "L" around 1/2" deep (this lets you mount to insulated standoffs to the board).
The hole spacing from side to side for the triac tabs is just the triacs sitting right next to each other so you can just lay those out on your aluminium. You want the triac back to be fully on the aluminum so the row of holes need to allow the heatsync to NOT stick down too far. The legs of the triacs allow a degree of up and down so the two standoffs from the heatsync to the board just needs to be close enough that the legs reach the board to solder AFTER the triacs are mounted to the heatsync and te heatsync is attached to the board.
See this thread: http://diylightanimation.com/index.php?topic=4513.msg76272 (http://diylightanimation.com/index.php?topic=4513.msg76272)