MacOS – How to pin output volume control to screen

I am looking for a program with a small window that allows me to adjust my computer's audio volume. I would like this window to be pinned over all applications/windows at all times. It would occupy a small area on the right side of my screen (for example).

I understand that I can adjust the output volume with the speaker icon in the menu bar by click, click, drag. I would prefer to have the ability to change the volume on the fly, with simply one click.

I understand that there is a dedicated volume decrease button and a dedicated volume increase button in the top row of keys on the keyboard. But using these buttons to set the desired volume actually takes me more time than setting the volume with the click, click adjustment via the menu bar speaker icon.

It may be that I'll need two separate applications to accomplish what I want: one application to adjust the volume, and another application to keep the window of the other application pinned on top of all other windows. The first application is what I need help finding.

For example, I tried an application called Volume Mixer. It offers a handy audio "mixer" that even allows you to simultaneously set and independently control differing volume values for different applications. But, it offers no method to keep the mixer dialog open; if you click outside of this dialog, the mixer dialog automatically minimizes back into the menu bar icon. And, because the dialog is not a true "window," I can't use another application to pin the dialog on top of everything on screen. I don't know of a program that is able to keep a menu bar icon pressed permanently.

The easiest option would be to just use the Sound dialog in System Preferences. But it is too wide and bulky; it contains a lot of controls that I don't want. It would occupy too much of my screen.

In short, I am looking for a simple, basic, unobtrusive, and compact volume control slider/knob/interface that I can permanently keep on top of all windows.

_{OS X El Capitan, version 10.11.6.}