When you cut with a hacksaw clean up the shavings
Heat the pipe slowly up the length of the slit
Line it up put in place and hold that bulge in till it cools
Could do with a touch up, we'll see how we go
Trim and shape the ends to open it up so you cam snap it back on
I don't know if this is necessary but trim the sharp edges as its not about blighting in position but shape
Take off the caps to the pannier mounts
Check the angle, cut, heat, line up and jam it in that hole and hold steady till it sets.