I want to propogate some 'Photinia fraseri' (red robin) and it says that i
need a cold frame.

I don't have room for one, but I have a window box with one of those water
reservoirs underneath which I have found very good for propogating some
cuttings before.

My question is; Is there any neat trick for getting some plastic sheeting
to sit over the window box please, to to create my own mini cold frame?

Also should I make holes in the plastic for ventilation or not? Thanks
for advice.