Since there are usually 2 lanes to warn, I ping mia and 'enemy champ-alive' from the tab, so they know which lane is missing without having to look at the map/minimap.
Now If I saw the direction they were headed, I also ping a trail of 3 danger pings halfway to my team mate(s), the 3rd one in their vicinity but not on top of em. that way if they're too focused to see where the danger ping is going on, the 3rd will be in their screen, but not necessarily ON them to annoy the fk outta them
Sadly enough it doesn't always work, but that does not mean ping more or spam chat. it's not worth spamming ppl and annoying em just for those few that simply don't get it...
imo
btw, stopping mid lane to type can get u killed