I did two for bolt hold open on the range. You need to solder or braze a piece of metal inside the end of the charging handle channel of the top cover. If I can find the link where I found out about it
I'll repost.
If so how did you do it?
I used to unscrew the barrel and leave it next to the uzi at the range (there's your empty chamber)
one day a rangemaster was all... "i still want the bolt opened"
I think for that guy, I'd slip the top cover off and pull the bolt out , put the top cover back on and walk past him holding the bolt in my hand and say "bolt opened" LOL!
That is what I do(pull the barrel) but I always tell them first when I enter the range.....never had a problem. I also have an orange plastic empty chamber "thingy" that I got from another member here for each of my UZI's. I have recently put one of them in each UZI case/box so I may start just inserting that if it's acceptable to the range monitor! Either way works for me.