Amazon has--or had, I may have snagged the last one--Fein Systainers for the Multimaster, apparently complete with insert, $115.

do you have the Amazon item number for what you bought?

Agree… it wastes too much space

Plus, you have to be very mindful about how you wind the cord, or the lid won't close right.
I had one, complete unit in the Systainer from Fein, that was the biggest annoyance. The cord is nice and soft, but it is in that "European" style of being very long, compared to US standards. You have to coil it up pretty tight for it to fit. Therein lies the beauty of Plug-it cables. None of my Festool machines have the cable in the Systainer, except the RAS115, which is my only complaint with it.

Standard 4m cord. Why shorter? The DE hose is 3.5m, so 4m is a nice length of cord. Machines with less... I'll either not buy or replace the cord.

The Plug-It cord I store in the Systainer with the machine. Only in my PS-300 Systainer that was annoying, but that is completely fixed now with my own custom Systainer for it. See here; https://www.festoolownersgroup.com/festool-and-tanos-systainers/show-your-systainer/msg632691/#msg632691

Coen, I couldn't tell you? It's just the way it is. I didn't say it was good. It's not like there is some specific length, they are fairly random, just shorter than 3.5m.
If I had to guess, I would say that it is probably a combination of someone thinking it is less of a tripped hazard and saving a few pennies per unit. Somehow that same line of thinking often requires an extension cord? Not the smartest in my opinion. I generally prefer the European standards or procedures. They seem to have been actually thought out. The only one I'm not such a fan of is the new power-interrupt thing. It is undoubtedly safer in 99% of cases, but not so handy for router tables.

I have 2 Plug-it cables out and ready for use. One on the CT at one end of the bench and the other hangs on the handle of my toolbox at the other end. Sometimes I use a few of the tools without extraction. The PS420 and sometimes OF1010 go without. This way I don't have to deal with the cables when removing/returning the tools and I get the benefit of having some of the cables totally un-used. The get no damage or wear that way. If something happens to one, it can be replaced with another that is 100% new. The sad thing is, when you have as many of them as I do, it's a whole lot of money, for effectively nothing.  I probably have 10 of them in a drawer and still use the original 2 that I left out.