I put two tiny drops of super glue on each edge and put the caps on. It holds them fairly tight. The only drawback is that you've to be careful where you're putting the glue and need to make sure that you don't break anything when trying to remove the cap.
Just put together another DIY (bought a few frmo cube4you..) and ALL of the center caps came off really easy, paper in them all and its going beautifully.
Actually, with the Chinese DIYs, I don't do anything with the center caps; with time, they just stop coming off. (Same thing with Eastsheen 5x5x5 center caps)
I can't take my center caps off my DIY, even if I take the surrounding cubies off it and try my hardest it won't come off. I guess it kinda works with only 4 washers, it pops a LOT, though. I locks up a lot too, but moves smoothly even without it being lubricated.
So my red side is tighter than all other sides, can't open the center cap. :x