Seriously tho, it was just an attempt at being brief about what we all should keep in mind -- aftermarket stuff is a gamble, and we should be prepared for it to screw up an otherwise perfectly fine gun, or at least require some fitting and tinkering.
I had an MP5k 3-burst housing that I got clipped and pinned from HKparts. Tried to fit a BFS binary trigger into it. Needed fitting. At the time, I didn't have a Dremel either. So I bought a knock-off Dremel and got the thing working reliably. Played with it, and got tired of out running it. Sold the whole thing.
OP - but I have found the 'Dremel' quite useful since for a bunch of things, most of which isn't gun-related. Never a bad thing to have tools around. A Dremel-type thing and a bunch of bits can be a real time saver for all sorts of stuff. Just sayin'.
Own: HK91, SR9, SP5k, G36k, HK416, P30L, 2x P8, 2x P8A1, USP9
Future: Done, until HK comes out with something new & nifty. Might start building an FNH collection.
As far as rubber goes I forget what I used, I found something laying around the house and cut a small piece and super glued it to the semi auto shelf block (the small ledge) That takes up the gap space between your shelf and the trigger pack. I've since sold that gun awhile back so I don't have pics of that one but can show you on a BPP gun. This one happens to fit perfect so it didn't need the rubber spacer. The rubber spacer I used was about as thick as two credit cards
I had an identical issue with a clipped & pinned lower for my ptr, purchased through hkparts. It wiggled up and down and actually wouldn’t eject spent rounds because the trigger box and thus the ejector lever rode too low. I added some JB weld metal to the steps on ears on the lower, and after it cured I took a small file to it a little at a time until until it was rock solid tight. Wiggle & ejection issues solved.