I looks like you are very limited and you would have to create something custom that extends the lens chasis.
According to this review
There is no lens hood available for this lens (Canon recommends using
a piece of cardboard to shade the lens). There are no filter threads
provided. And I can tell you that extra care is required to keep that
bulgeous glass pristine. Strong recommendation: the supplied and very
protective lens cap (shown in the product images below) should be in
place unless the lens is actively being used.
The angle of view and the usable image circle (more later) from the
Canon TS-E 17mm f/4 L Tilt-Shift Lens are such that a lens hood is not
practical - and one is not included. Included is the unique
bayonet-mount, wrap-around protective plastic lens cap - with a strap
(The manual warns against carrying the lens by the strap - thanks).
In terms of custom you could buy one of these and chop the end off - and extend past the glass
UPDATE: According to this site - under TSE Lenses a L lens guard cap covers this lens