The valves I used I got from an aquarium supply shop, then on ebay.
You can find them here: http://www.watergardengems.com/SUBcategory.php?categoryKey=333&side=aquarium&subC ategoryKey=334&subCategoryName=Valves
I got the 3/4. The key is using the swing - not the spring valves as they do not restrict water flow. They say they need to be perfectly vertical, but I've found them to work fine when not perfectly aligned.
I'm surprised about your bags filling under motion though. On my bags, I have the fill line start at the hull, run to the transom, up the transom (behind the cosmetic divider wall) and to the top of the interior of the boat. Then into the V-locker compartment and back down to the bag. At all times there is a good portion of the fill line above the bag to prevent backflow.
Of course the fill line goes through the aerator pump at the hull as well. I get no accidental filling - it seems like it would be next to impossible going through the pump and up over the bag. I do however only have a mushroom, and not a scoop-strainer.
Jon - is your fill line routed like this into the top of the bag? Or is it routed differently/into the bottom of the bag/etc?
I did read somewhere that folks using a scoop-strainer drilled holes in the back of the scoop strainer so it functioned more like a mushroom. Can't recall if it was for this issue or something else, but that would cut down on the flow.
You could start with a hole or two in the back, add a few more if you still have the problem.