Have you ever looked inside a macro tube to examine the optics there? haha. No optics, it is just a big tube filled with air.* This is why it is an extremely easy device to replicate. Maker Vinnie Hirt used the macro tube quality of nothingness to build his own set of an uber extension tube for a mere $5 give or take.
For his Micro 4/3 Hirt used 10 centimeter of a 1.5″ ABS pipe along with a rear lens cap and a camera body cap (both can be bought on eBay for cents). He also used an old manual Canon FD lens and a lens adapter which he already owned. (Using a manual lens makes life easier as you have access to the aperture ring).
Now here comes the magic all you need to is glue the rear cap and the front cap to the pipe using JB weld (or similar) to provide mounts on either ends of the pipe (to the camera and to the lens).
With the micro 4/3, 10cm will give you about X2 magnification and will cost around 3 stops of light. If you need to adjust those, you can get some help from the Macro Extension Tube Calculator.
Here are some of the shots taken with the tube:
*higher quality macro tubes will have electric contacts to control lens aperture, and focus / focus feedback.
[DIY Extension Tube | Makify]