Hey Madmarc, I designed and developed the SwayRocker system. I am in the process of developing an upgrade for this. its a bit challenging, but hopefully I can have a solution soon - so ill keep you updated on this...seems a few peope are keen to see something similar. Our system uses any standard old dumbell weight, which mounts inside of a routered 'pocket' under the deck to make sure the top is clean and not cluttered. There is space for 2 weights, so you can mount up to 5kg (2x 2.5kg) in order to balance things out. They are also adjustable, so you can really fine tune this to suit your specific trainer setup and balance point.