For £5 it wasn't worth the hassle, but if you can't get them anymore, then a washer would work. The only issue would be if you get washers that are too big or not strong/thick enough to support the stress of closing the bonnet strut.
Not sure a threaded plate would be any good as the plates are quite thin, too thin to get a good grip on a bolt so would need some sort of nut on the back, be it captive or not.
As for the plates above, they 'look' ok, but would need to check measurements, but you woould only need 2 with captive nuts, you would still need two without