It's not so hard to create a keyboard layout using the Microsoft Keyboard Layout Creator (MSKLC) [0]. I don't know if it can do everything that uncap can, but I'm pretty sure it can do most things, and it doesn't need a separate program to be running, because the keyboard layout is just installed as part of Windows.
A number of my favourite keyboard layouts are at http://keyboards.jargon-file.org/ (notably Pseudo-VT320, which turns AltGr into a Linux-like compose key).
It looks a bit old, but works fine under Windows 8 and 10.
[0] https://msdn.microsoft.com/en-us/globalization/keyboardlayou...