Contrary what the topic says this is not fully DIY. The steel adapter is made by fellow hobbyist called ferri. And the carbon fibre tube is from Ebay. It took some filing and Dremel’ing to make the follower pass the steel adapter smoothly.
This came to replace my current barrel-length aluminium magazine. This weights approximately 25 g less than the aluminium magazine and total weight without spring is around 150 g. I seem to have forgotten the magazine support from the picture. And I don’t have suitable spring in hand so no clue how many cartridges will it hold.
Steel, carbon fiber, plastic end cap and some 2-component glue to hold it all together. Total costs 30 €.