Hi Greg,
I've built a few stackpacks and lazyjack systems for people, including myself, and prefer to use 1/4" three-strand dacron for the jacks.
I lashed a teardrop stainless thimble a couple feet below the upper eyes of my lower after shrouds, then a plastic cleat down at a serviceable level to make the lazyjacks to:
Photo of thimble (I don't want any crap for my ugly lashing...it was night and post uh...dinner...and its beverages
Photo of cleat
The 1/4" dacron runs up from the cleat, through the thimble, then down to the stackpack with butterfly knots tied in three places on the main lazyjack line. Then two sections of line rove through the butterfly knots and tied to the stackpack at the appropriate length to support it in four places, with a single length supporting the front. So the two aft-most legs are one line rove through a butterfly knot (eye), and the two middle legs also one line. In this way you can adjust the tension by tightening the legs, or by tighting the entire stackpack's lazyjack line:
Photo of lazyjacks supporting stackpack
Photo of lazyjack line made off to cleat on shroud, and stackpack.
I do not think it is necessary to use Dyneema for lazyjacks, unless you were going to have the lazyjacks support the boom as a topping lift. My polyester three-strand has been on the boat for three years and is doing a great job. Plus, the whole lazyjack system cost about $20. Though mine is setup for use with a stackpack, the concept is identical for a standard lazyjack rig. Also, after some tweaking of placement of the butterfly knots, I don't have trouble with the roach of the main getting caught in the jacks, as long as the boom is sheeted into the wind.
Cheers,
Aaron N.