Way too late to help you probably, but as a total novice, I just bought a Zippy 90. The sponge does not have a metal or plastic bar - it has ribbon on each side of it and slots into a channel under the machine.
I couldn't get the carriage past the needles in the B position as some would stick up and get caught. The magnets are strong, but if the sponge is ok, it will stop the needles from sticking up.
The sponge was rotten and sticky and I could not get a replacement. After cleaning and oiling the machine, I used thin flat sponge cut to size (1 cm wide) and flanked it with tape, using Power Pritt Gel which is solvent free and does not affect the plastic.
A minor adjustment was needed to make the sponge lie flat all the way along the channel - I simply pulled both ends at the same time to flatten it, that's all!
Yes, it now works fine with a totally hand cut piece of sponge. Nothing complicated or fancy. No need to remove magnets which are necessary for the machine to work properly.
Hez, July 2011