At the races I've worked recently with Indy Car, they use flashing yellow lights at key places on the track to supplement the yellow flags. On a full course yellow, Race Control activated them, but for local yellows, we had a remote and we activated them, then [hopefully!] turned them off when the incident cleared. (We worked with each other to help make sure the lights did get turned off when they should - the next station would say "Turn 4, your light's still flashing!") It wasn't perfect, and initially I was sceptical, but I must admit, it really does help the drivers see the yellow area, particularly in bad weather. But of course, Indy Car has big fancy lights and remotes that IRDC obviously couldn't afford! Earlier today when I posted the link I had just done a cursory Internet search. You're right, 4.5 inches is probably too small, but perhaps there are bigger ones for not too much more? I'll do a little more browsing ...
I know Westwood had "deer lights" which were activated when wildlife was spotted trackside. I remember being assigned to "Deer's Leap" for that task in my first year of flagging (which was Westwood's LAST year), but I can't remember how the lights were turned on or what they looked like. I just seem to remember sitting on a platform trackside keeping a lookout! (Very boring but necessary job.) In those days, it wouldn't have been by a remote control device - maybe wired with a toggle switch? Nick or Roger will remember, I'll ask them. And I wonder what the rule was when the lights were activated ... no passing, or just warning you of the danger and you're on your own? Of course, I imagine at PR nothing could be attached permanently to the drag course wall.
I do like the idea of somehow implementing an "extra" warning though, at least until the problem is fixed.