I’m a big fan of sealed rum beds. I have built 5 Lightbicycle wheels like this and have a couple of Nextie ones waiting for me to get time to build them.
I use a small section of mild steel spoke thread tied to nylon thread.
Drop that in through the required nipple hole and use a magnet to pull it out of the valve hole. Thread the nipple on, pull the thread and it pops out of the nipple hole. Screw on a spoke or as I do, secure with a miniature clothes peg and move on to the next one.
I’ll normally do all this while watching telly. It can be a fairly chill and satisfying process.
One tip, start opposite the valve hole and work back to avoid pulling new nipples over those already installed.