Lens hood stuck

[Buttkicker]

Hi my
Canon 400mm F5.6 Lens hood has got stuck about half way out its not a great problem but if I ever sell the lens I will obviously have to get it sorted
the black rubber seal has come out (got stuck) in between
and obviously needs a new one fitted

does anyone know is this possible to fix (new rubber seal) here at home or am I am eventually going to have to send it to canon

obviously prefering option one

for those not familiar with this Lens the hood is permanantly attatched to the same as the 300mm F4

another question is it possible for me to buy the rubber seal and some help with fitting it would be a great help
is there an online diagram of the internals of these Canon Lenses
regards
Chris