Van's sales a VA-195D Access Hatch and a VA-195C Access Hatch Doubler for an access hole to be cut into the bottom skin of the RV-10 Wing for access to the stall warning.
You might consider ordering in such items and looking at the RV-10 plans (located at
http://www.myrv10.com/Plans/WingPlans.html) scroll down to Section 19.
I don't remember the exact size of the hatch but it is big enough to get you hand inside and do some work.