Before you give up completely on installing it where you really want it, I have a potential solution. The thought of having the controller dangling off the inspection panel every time you try to remove it would drive me mad, so I have taken some pictures seen below and written up a brief work instruction.
Because of the pressed indentation in the rib, you need to space the controller out on shims regardless. Cut some shims out of some 0.063 sheet, drill for a #6 nutplate and countersink the 3/32 rivet holes locations. Temporally shim and clamp the pitot heat controller in position and drill the #28 holes using a right angle drill attachment. Remove the pitot controller, then use your nutplate jig to drill the 3/32 hole locations to attach the shims and nut plates. Install the shims and #6 nut plates using some 3/32 countersunk blind rivets. I actually fay sealed mine first using some sikaflex pro and left shim and nut plate assembly clecoed in place for a day so that the clecos could be removed and weren't in the way when I riveted. Mount the controller using some #6 Allan head or torx screws.
Good luck.
Tom.
