Over the 34 years we have been involved in Sports Reenactment, a lot of gear has come across our field. We have tested and experimented with everything from wood, aluminum, plastic and foam shields. The sides have been open, filed, wrapped in padding, vinyl tube covered, cloth and tape and even foam covered.
With all of this research backing us, and our examinations of what the rest of the community is doing with their gear, we have ultimately come up with what we consider a great system for safety.
We now know that the surface of the shield matters less if it is metal, wood or plastic, so long as it is not jagged. The primary reason for this consideration is to allow the weapon gear to last longer. It has very little to do with safety for combatants being shield bashed as the area of impact is spread evenly.
The sides need sufficient padding, once again to protect the weapons from getting broken down hitting the edges of the shield. There is a slightly greater chance that the attacker might hit his hand on the edge of the shield, but that is a considerably rarer situation than some might think.
In the end the heavy restrictions on shields we once had, such as padding the front or covering them in cloth, became unnecessary restrictions. when done right, a Melee shield can have minimal padding and coverage and still offer superior efficacy and safety.