I have built a couple of 2m risers a long time ago, my first real thermal was with my original transparent red one. A memory that will always stay with me! I too have built several sig kits, all the same sig modeller friendly. With those experiances, I often have been tempted to try a riser 100. If the 100 plans include instructions for spoilers I would lean that route.
You are allready dotting your Radian, You might as well go standard class (100"), or open (3m, or larger).
If you go for the original riser(2m), don't be afraid to stretch it a little, try 2" fore, 4" aft. Snuts