@robertvb83 Thank you too for the elaborate answer, I may have similar decoders (if they show up as DragonRise in Retropie), but I never felt compelled enough to test their special ports.
I can absolutely understand your reasons for forgoing the door (yet). That said, I can recommend magnets for the back door. They can be mounted easily, and it would spare you the screwing every time.
The speakers are not that important. I am just curious what parts other builders use. So, you don't have to look them up just for me, but feel free to post their information here if you happen to stumble over it sometime.
I first tried some cheap speakers, too, but then I decided to go for reasonable higher quality and spent 70 € in total for these:
My mini aplifier even has an output for a subwoover, so I may add one to the cabinet someday. :)