Their should be a screw with a washer on the rubber drum. After putting on the sand disks just tighten the screw. That compressed the drum forming a tight seal. The type that have an adhesive backing on them that fit on the flat disk, just press down on the disk. If the rubber disk is dirty they will not stick so wash with windex and let dry. This type will not take a lot of pressure.
Berny
Phormer Phantom Phixer 
On the bench
TF-102A Delta Dagger, 32nd FIS, 54-1370, 1/48 scale. Monogram Pro Modeler with C&H conversion.
Revell F-4E Phantom II 33rd TFW, 58th TFS, 69-260, 1/32 scale.
Tamiya F-4D Phantom II, 13th TFS, 66-8711, 1/32 scale. F-4 Phantom Group Build.