Did one of mine some years back removed the acrylic window stop screws at ends of the hinge chanel and slide the window off the channel. Took off the window catch plates that the window locks on to and the stabilising arms and pulled off the plastic strip [replace the stip alos if it looks it needs it] around the frame, then removed the rubber seal.
Either wait until you remove the seal and measure it adding six inches. [You can always duck tape a plastic sheet over the opening if weather turns nasty.
Try your local caravan dealer who has workshop they may sell you the rubber seal or tell you where to get it.
Good luck and it's not a hard job to do.
If you have problem re-fitting the screws of the catch plates in and they don't go tight and keep turning, I solved this by opening the hole for a small rawplug tightish fit and problem solved.
Bob
May 2009
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Try fixfault.com to fix caravans/motorhomes.