New answer:
I updated my package to support not only templates but a menu with arbitrary actions so making an un/comment shortcut should be easier now.
Follow those steps:
Install/update DevTools` v0.10.0+
Actions are available in DevPackageDark.nb
stylesheet but you can make them work in any notebook by:
Needs @ "DevTools`";
(*Keep in mind it hijacks "Ctrl+," which stands for NewColumn *)
NotebookActionsEnable[]
Open user actions file with EditNotebookActions[]
Put there a new item:
<|"Label" -> "Un/Comment",
"ShortKey" -> "c",
"Action" :> Module[{foo, nb = EvaluationNotebook[]},
foo[RowBox[{"(*", " ".., content_, " ".., "*)"}]]:=NotebookWrite[
nb, content, All
];
foo[RowBox[{"(*", content_, "*)"}]]:=FE`toggleComment[];
foo[boxes_]:= NotebookWrite[
nb, RowBox[{"(*", " ",boxes, " ", "*)"}], All
];
foo[$Failed|_CurrentValue]={};
foo @ CurrentValue[nb, "SelectionData"]
]
|>
Click 'Save&Test'
You are ready to go with Ctrl+, (NewColumn hijacked) followed by c

OldAnswer:
You can use LiveTemplates Live code templates.

Alternatively you can create a palette with something like this:
Button["c"
, NotebookApply[InputNotebook[]
, RowBox[{"(*", RowBox[{" ", "\[SelectionPlaceholder]", " "}], "*)"}]
]
]
Or try to edit KeyEvenTranslations.tr file. Search around for guidelines but I find this method the least user friendly.