We know of no solution that works really well. Some systems keep out leaves, but as you suggest, allow smaller objects such as seeds to get caught in the protector, or get through into the trough.
After they are installed, most systems make cleaning the eavestroughs or gutters more difficult. Also, most do not allow sufficient water into the trough during periods of heavy rain. Under these conditions, many systems allow too much water to flow over the protector and onto the ground, rather than into the gutter.