rigid, rigged, ridged

Rigid or rigid means stiff, inflexible, or unyielding, literally or figuratively--
The rigid wooden pole snapped in two during the storm.
He followed a rigid code of behavior, and wouldn't accept any new standards.

Ridged or ridged means having a ridge, meaning a narrow elevation or raised strip--
It had a ridged back, like a wild hog.
We may need to flatten the part of the roof that is ridged to may fix the leak.
It's on the other side of that ridge, on top of the first hill.

Rigged or rigged is the past tense ( yesterday, or some time ago ) of "rig", meaning to put in proper order for use--
The ship is rigged and ready to sail.
He rigged up a portable generator on the spot.
Rigged may also mean furnished with equipment or gear--
They rigged him out for a long camping trip.

Now that you know that, you can say--
"He rigged one end of the sign to a rigid stake, and the other to the edge of the roof, where it's ridged."

Note--"rigor" means strictness, stiffness, or severity, figuratively--
He disliked the rigor of life in a religious order.
Not to be confused with "rigger", meaning someone who sets up equipment in working order--
He worked as a rigger on a large sailing ship.
Note # 2--"rigorous" means characterized by harshness or severity, figuratively--
Only rigorous discipline kept order in the camp.