45cm+ trickle vent needed for double glazed wood windows!

[gixer1000]

I need 4 internal trickle vents for wooden double glazed windows. The ones I need seem to be of an awkwardly large non-standard size because there are 2 cut outs in the frames. As you can see from the pic, the vents will need to be about 45 cm or more. There are lots of 40cm vents out there for a reasonable price, but as you can see from the pic below, they will not cover. The larger 50cm+ vents are harder to find and cost about 3 times as much as a 40cm vent! Does anyone know where I can get cheap vents that will fit?

 

[freddiemercurystwin]

http://www.handlestore.com/cart_view.php expect you've seen these

 

[gixer1000]

These are the only ones I could find anywhere, but they are so expensive

 

[denso13]

Maybe these http://www.reddiseals.com/acatalog/window_hardware_trickle_vents_slotted.html