Mistwell
Crusty Old Meatwad
I do have two questions to those who say that a shield is similar to a mace.
1. Do you allow the attack to be made with proficiency bonus (with no feats)
2. If you don’t allow proficiency bonus why don’t you?
I think for me, I'd allow mace damage but no proficiency bonus, unless the player made a compelling enough argument about a particular kind of shield which is so similar to an actual weapon that it should allow proficiency.
I see three types of improvised weapons in the rules.
1. An improvised weapon so similar to an actual weapon that it can be treated as the actual weapon: use proficiency bonus and damage as if it were that weapon.
2. An improvised weapon which bears some resemblance to an actual weapon, but which is not so similar that it should be treated as the actual weapon. No proficiency bonus, and damage as if it were that weapon.
3. An object that bears no resemblance to an actual weapon. No proficiency bonus, and 1d4 damage.
A shield falls into category 2 for me, unless a specific shield is successfully lobbied as being category 1 or 3.
As an example of a shield which I think should be treated with proficiency (though not for mace - maybe short sword) I am thinking something like this:

And this one
