Caravan roof light wanted

[#1]

We own a BK Caprice static caravan, now out of production, and need a replacement roof light for a gap measuring 400mm x 400mm, or 14"" x 14" in old money, in the shower compartment..  It's not one of the current standard sizes so I haven't been able to source a replacement, and wondered can anyone in this forum help? 

[#2]

Just put in .... "Caravan Roof Light ".. into the ebay search box and the second item displayed is this.....
http://www.ebay.co.uk/itm/Roof-Vent-Sky-Light-Caravan-Motorhome-400-x-400mm-Flynet-Beige-MPK-Rooflig...

[#3]

Been there, done that, but they're quoting the size of the opaque component on top.  Ours blew off in a gale earlier this year so I don't have it to measure, hence quoting the size of the gap, and it appears that isn't one of the standard sizes currently in use.  
I appreciate your effort.
dick:quote

[#4]

So, can you "modify" your roof light hole size, either to enlarge it, or reduce it, to make the best size currently available to fit?
I.e. cut the hole bigger, ( if needed) or reduce it by fitting a square collar prefabricated to the size of the new fitment, and secured by using suitable fixings and sealant?

[#5]

@ Shutter
I've got exactly what I was looking for from someone on another forum.
Thanks for your time, much appreciated

[#6]

OK... Good move... I had similar problem with a broken skylight... bought some "FABLON" clear, and just stuck it on top... stopped the rain coming in... !.. then a couple of weeks later sold the van ! ..