Okay, I gave it some thought and here's how it could be done.

If you wanted some sort of slip or failsafe device, you could forego gluing the top pieces. Drill a hole in the side of the top T-fitting and then another one in the outside top 90-degree fitting. All fittings should be sized for an easy, slightly loose slip fit. Now run a threaded rod from the inside hole to the outside hole. On one end put a lock nut, on the other, a washer, a coil spring, another washer and then a wing nut.

By turning the wingnut you can adjust the tension on the pieces thus setting a pre-determined slip point whereby any collision between the motor and an object would cause the pieces to slip, thereby lessening the chance of any damage to the boat.

I just cobbled one together and it appears it would work just fine. I'd have to take it on the water and purposely run into a few things to make sure, but from what I can tell in the shop, it appears to do what you're after.