I have found a better alternative to sponge on the sponge bars. I have replaced the sponge with a row of 12 conical springs, the type found in AAA battery packs purchased from Maplins Electronics. A battery pack costs 89p and has four springs. Remove the springs are attach to the sponge bar then cove with a length of metal tape, the type found in retractable tape measures. The smaller type are usually 1 metre long and the same width as the sponge bar. Purchased from our local hardware store these are £1.29 for a twin pack.
When the tape is cut from the holder remember to put Sellotape over each end, they are very sharp. You just have to remember when inserting the sponge bar it has to be inserted upside down so that the springs are facing away from the needles. I use this method in both the main bed and the ribber attachment. The bar works perfectly and I have been using it for over a year now and the springs are still in perfect condition. Up until I created this method the sponge bar was the absolute bain of my life, very hard to find, expensive and messy to replace on a DIY basis. I hope this helps.