I messed up the proper sequence covering a J3 and installed the interior fabric before fitting the new boot cowl. Mild panic for a while trying to figure out how to find the hidden u-channel and drill into it to attach the cowl. I stacked two rare earth magnets together, about the size of a quarter, and sure enough magnetism still works. On a visible u-channel, the magnet went to the center, but through the cowling it was consistently on the bottom of the channel. I used my cell phone to look behind the cowl to view the first clecos to see the position. Patent pending.