I was looking for foam rubber vacuum suction cups for a while with an ultimate goal to enable naminukas to climb a brick wall. I am still not sure if that is possible as I couldn't get hold of foam rubber suction cup of suitable size and without heavy metal plate. To convince myself if it is the direction worth pursuing I have build a suction cup from a cheap self adhesive neoprene foam rubber sheet from eBay and 3D printed decommissioned naminukas' parts:

Cut with a hobby craft knife it doesn't look nice but it is good enough for testing. It does stick well to smooth and hard surfaces. It also sticks to pavement tiles and some outdoor concrete surfaces to which regular rubber cups wouldn't stick at all. Holding force is not enough to lift a robot but it is also smaller size than I would like to use. So while this cup doesn't stick to bricks at all I think overall this design looks promising. Next step is to test larger diameter cup and softer rubber foam if I can find one.