I have also used paper, taped into a tube, rolled snug around the lenses. These methods work fine if the doublet elements are the same size.
You can also re-assemble the doublet without cement, as long as you keep the surfaces clean, you should see a newton rings pattern reflected after assembly, which will tell you how symmetrical the elements were aligned. You will loose ~4% transmission at that interface, but the lens will otherwise optically behave correctly, even with the visible newtons ring pattern. However, if the original lens separation is clean (no fungus or stains), then you may just as well use the camera as is, as you only loose ~4% transmission where you see the separation. You really should re-cement them if you take them apart though.