slotted foam works for me as well rather than flat or rippled - however........as per a previous thread, i really just dont get the viewfinder concept, and think its a gimmick that will eventually die a death. To me, the whole point of having a swing leaf is to introduce four sides for mounting flies within the box, so why bother when you no better off if youve still only got two surfaces? it just makes the box a lot heavier, and they are heavy, just to save all that immense physical effort & mental strain it takes to open a plastic box to see whats inside

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Jeez, if its that important not to open the box to know whats in, just stick a label on it!
Personally id avoid 'viewfinder' type boxes and go for a proper swing leaf with 4 sides & slotted foam, like a C&F, or better still get the platinum tackle versions advertised on here for a 1/4 of the price, I can thoroughly recommend them.
I you still want to go for the viewfinder, you can have mine for a few quid, as i fell for the gimmick myself, until i used it/sat back and thought about it a bit!
Just my ramblings for what theyre worth....
Rover