Where to cut duct for o-rings?

Doing the heater matrix o-rings this weekend, just wondering if anyone has a pic of where abouts to cut the plastic duct, or is it obvious where to cut when you get down to it?

Shopping list for job:
stc3262 x2 o-rings (quite important)
duck tape
M5 allen cap head bolt 40mm ish and nut
Ball ended allen key to fit above
Finger plasters

Have I missed anything? Any advice appreciated

rangerovers.net guide and drill a secret hole