Going to have to build your own cricket.
DO not install that chase within 2' of the peak of the roof.
If that was mine I would remove the shingles where it's going to go. Mount it to the roof (really need cleats made out of 2 X 4's on the inside with predrill pilot holes for 2-1/2" ceramic coated decking screws.)
Cover the whole thing with house wrap or tar paper, reinstall the shingles using step flashing, install counter flashing.
I then would use 1 X 4 vinyl lumber around the , top and outside edges as trim. This will get the siding up off the roof for less chance of rot. Siding needs to ne at least 2" up off that roof.
On top of the bottom trim you then install Z moulding.
I personaly would never use that type siding for this. If you do it really needs to be preprimed and painted on all sides and also seal the ends where the cuts are.
I'd use James Hardee lap siding instead. If not that then concider using Hardee panels that would be installed horizontal before the trim went on. That way it would look almost the same as those boards you want to use but would never rot or have to deal with peeling paint all the time.