In many cases you can buy plinths in online stores but often they are rather expensive. If you want to make your own plinth it’s a good idea to use plastic cap from some bottles and small containers. In bathroom you can find some useful plinths like cream caps or deodorant caps. Just find a good size and let’s start.
I know that it’s very simple to make plinth this way, I’ll show you more difficult way (because I found hard clear plastic container that fit perfectly under 40mm round base). Also  here you can buy this containers – Agtom.

I have to remove thread from container, it will be really hard without mini grinder. Any time you start working with tool like this wear safety glasses, you probably don’t want lose an eye :)

Now I remove excess with pilers.

And polish border with grinder.

Now  I paint black inside because I don’t want have clear plastic, it will help to get solid color outside.

This white sheet is made of thin plasticard, now it’s a good idea to place it on bottom of plinth it will secure black paint inside and mask inequality of border.

Some super gule on border.

And place it on plasticard.

Cut off excess with scissors.

Polish edges with sandpaper.

And paint black. It’s ready to use.

Here is result, quite nice display bases – clean and simple. Of course you can find better caps non-transparent, without thread and easier to prepare as plinth. Hope that this tutorial is useful for you! :)