I just thought I’d pass this on for anyone who may be frustrated while trying to get spoke nipples into double-walled rims.
Instead of spending an hour trying to shake lost nipples out of the void, just use a Klein Tools Vaco K34 screw-holding driver to set them in place. You just slide the sleeve toward the handle, hold the nipple on the tip of the driver, then slide it forward again to lock it in place. Then insert into the rim, get a spoke on it, and release.
You can find one pretty much anywhere for less than $10.