Microsoft Keyboard Layout Creator
The Keyboard Layout Creator lets you create keyboard layouts. You can add any Unicode character to your keyboard layout. You can create dead keys. The keyboards cannot be context sensitive like Keyman keyboards can. The program is a free download and the keyboards you create can be shared with anyone. The keyboards are installed into Windows, and then appear in the list of available keyboards in Control Panel.
You can choose any Windows keyboard as the starting point, and modify that keyboard to add the characters you need.
It will work in Windows 10, even though the website doesn't list 10 as a supported version.