Plastic 'flight stands'

I find the various kinds of plastic flight stands for star ships, VSF
ships and (potentially) GRAV/VTOL figures to a lesser degree are
unstable
(larger figures regularly can be tipped over by contact.)

Short of replacing them with metal stands what do you do to prevent
this?
 I can see filling the bases with something (but what material?) or 
gluing them to metal washers (would this be sufficient?) but if anyone
has an alternate idea please speak up.

Gracias,
Glenn

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