Automating constructor definitions (OptionValue eval time)?

Question

I'm settling in on a programming pattern for "constructors." I can illustrate the pattern with the following example, which defines a constructor newBar for "objects"-as-Associations of type bar:

ClearAll[bar, newBar];
Options[newBar] = {"a" -> 42, "b" -> 43};
newBar[OptionsPattern[]] := <|"a" -> OptionValue["a"], "b" -> OptionValue["b"]|>

?? newBar

Global`newBar
newBar[OptionsPattern[]]:=Association[a->OptionValue[a],b->OptionValue[b]]
Options[newBar]={a->42,b->43}

 newBar[]

<|"a" -> 42, "b" -> 43|>

 newBar["b" -> 117]

<|"a" -> 42, "b" -> 117|>

This does a nice job of supplying default values. The only downside is that there is LOTS of repitition in the boilerplate, and I'd like to abstract that out. Here's an almost-working attempt:

ClearAll[capitalize];
capitalize[string_String] :=
  With[{chars = Characters[string]},
   StringJoin[Prepend[Rest[chars], ToUpperCase[First[chars]]]]];

capitalize["foo"]

"Foo"

ClearAll[camelback];
camelback[strings_List] :=
  StringJoin@Prepend[capitalize /@ Rest[strings], First[strings]];

Symbol@camelback[{"new", "foo"}]

newFoo

ClearAll[constructor];
constructor[nym_Symbol, attributes_Association] :=
  With[{keys = Keys[attributes],
    values = Values[attributes],
    cNym = Symbol@camelback[{"new", ToString[nym]}]},
   ClearAll[cNym];
   Options[cNym] = Normal[attributes];
   cNym[OptionsPattern[]] :=
    Evaluate@Association[
      MapThread[Rule, {keys, OptionValue /@ keys}]]
   ];

constructor[foo, <|"a" -> 1, "b" -> 2|>]
?? newFoo

Global`newFoo
newFoo[OptionsPattern[]]:=<|a->OptionValue[a],b->OptionValue[b]|>
Options[newFoo]={a->1,b->2}

So far, so good. It looks very similar to the bar example. The most notable difference is the <|...|> notation instead of the call to Association, and that seems to be the clue, but I can't interpret the clue. In any event, it doesn't work, presumably because OptionValue doesn't get evaluated at the right time.

newFoo["a" -> 42]

<|"a" -> OptionValue["a"], "b" -> OptionValue["b"]|>

Any advice?