After a couple of years of using these shakers on a nearly daily basis, I’m on my 5th shaker. The old ones still work, but they get harder and harder to clean as time goes on, so I wouldn’t worry too much about getting an extra one, as you may well have use for both in the long run (though I hope they’re gonna start selling that steel one the CEO posted about on Twitter a while back, because it seems to stay much easier to clean).
But that’s not the answer you were looking for, so, hmmmmm…
Perhaps you could sacrifice a strainer or a colander, and modify it to fit snugly in the glas jar. The newer shakers seem to rely on turbulence created by a bunch of holes to mix things. I don’t know how hard it would be to recreate that with a DIY setup though.