So yes, I think I figured out why I can't get the Peecock to work. But I would like some insights on how it might work out after all.
So when I first got it, it all went fine. Ofcourse testing it in the shower and everything. Then I just tried it every evening, because I wouldn't always wear it (not out at school/internship) and that went fine. 'Till at some point the holiday started (meaning I would wear it all day), and I just ended up peeing all over myself time after time. Which really depressed me.
Now after lots of weeks of no practice because I was hiking and such, I started with practicing again, and I think I know why it's not working for me. (stay with me a little longer)
I wear mine with the harness. Which is overpriced, but works really well for me. No extra fabric so less warm, holds it in place, comfortable, secure and everything. Also, when one sits down on the toilet, you don't have to hold the packer or pull the packerharness down as well. You can just leave it in place and do your business.

If you're not sure how to harness works, please see the website for more pictures.
So the shaft goes through the ring and you position it nice and low making it a comfortable packer. Then you need to pee, meaning you need to pull it a bit more down so you can 'cup' your downstairs and catch the pee. But by doing so, you force the harness down with the packer.
Problem is, that for the harness to have a nice snug, comfortable fit, it's a bit too tight to be pulled down nicely. This in combination with the nice and sturdy ring means.. (here it comes!) the bottomside of the ring pushes against the bottom side of the shaft, this 'flattens' the shaft a bit, which closes the hole inside. So with the 'pee-hole' closed.. yes, it backs up and you pee all over yourself.
So yes.. that's my theory, that's what's happening to me. I haven't figured something out yet, hoping someone might have a brilliant idea. And just for the record, wearing it without somesort of harness is no option because it's just not secure enough for me.