The nails were just there as temp solution to try and tighten up the lid more without getting out the wrench. But in reality an under cabinet canister and lid should be strong enough to only require a few hold down supports at most. If a canister lid requires more due to deforming, its simply isn't strong enough to take the pressure.
I don't mind admitting that mine was the same way working great for months next to and only 7 inches below the tank "top" waterline (only 2 PSI), but would start to leak if placed below 15 inches of the "top" waterline, and terribly hemorrhage water if placed on the floor 50 inches below the "top" waterline.
So I have no doubt your canister will work next to the tank, just like an HOB requires no pressure seal. But a floor canister below a tanks requires the ability to support all the water pressure that tank exerts, just like the tanks itself.
http://www.aquariacentral.com/forums/showthread.php?t=202122&highlight=diy+canister
http://www.aquariacentral.com/forums/showthread.php?t=191499&highlight=lock&page=3