You can use foam, springs, tubing, washers, etc ... if you just want the "look".
But if you're one that believes that hard mounting a pickup effects tone, than I would think all of these methods would be pretty pointless.
I fabricate wooden pickup shims that help raise the cavity to the proper height so the pickups can be mounted directly to the wood.
Just place masking tape over the cavity, trace the outline, and use this as your template.
Thickness may vary from one guitar to the next, so a mock-up is always a good idea.
BEFORE:
AFTER: