how are you using the blue filler mod ?? There are so many configurations out there its hard to tell which one people are using when the say blue foam mod, lol.
When I used the blue foam plug mod (foam only, no straws, staples, etc etc), I'd cut the width just barely wider than the rim of the 510 cart.
Then for length I would fold it in half and put the folded end it and make sure it was at the bottom of the cart and cut the loose ends flush with the top of the cart.
I would then pull it out and reverse it so the loose ends went in first, cut a 'V' in the fold (in the direction of your atty bridge when installed), then again make sure the two loose ends are now seated all the way into the bottom of the cart.
For a first time fill, I would again remove the foam, put about 5 drops in the empty cart, then gently replace the foam and seat it again (losse ends first) to saturate the foam a bit. Then finish off by topping it up with slow drips until you notice the fold 'V' cut is wet *but not pooling liquid", and you should be good to go.
Most times it takes some trial and error to get just the "right" thickness cut to ensure its neither too loose (flooding and leaking), or too tight (where it won't wick properly).
I've personally moved onto the ZFM (Zero Filler Mod) with the latest "H" cut silicone plug now and absolutely love it since I got it working properly. You can read about it here:
Zero Filter Mod
Hope that helps.......
VapeOn,
Greg