I went to Home Depot and bought two pipe caps, in the appropriate size, which I don't recall.
Get them hot, and put a good coat of wax on them.
I put a bead of RTV silicone on the ears and stuffed them in the holes.
You'll need to bend one ear down to clear the axle nut locking pin.
Once they're on, you don't need to take them off, 'cause the axle nuts will clear them.
They cost something like $2.50 each.