For spacers between the top and middle plates we're using a few scraps of a 10mm x 10mm stick we had sitting around. We had some elaborate plans of bolting everything together so we could get to the PDU if necessary. It turns out we forgot to get 1-1/2'' bolts, so we decided it would never be necessary to get to the PDU again and super-glued the spacers and plates together.
The beloved Arcticopter V served as parts donor, but will soon see the sky again as Arcticopter VI! It's still the best flyer in the fleet, I think because it's built so light.