I have the same problem with the standard 510 atm actually, I tried ZFM and it worked initially, but then leaked all the juice out.
I think we both need to continue to look into what fillers are available and try them out.
I have a feeling that polyfill and a cut straw (shortened so it wont tough the bridge, but long enough to help retain the juice due to that physics effect, you know the one im talking about.. 0

) could work well.
Keep trying different things, I find that my 401 carts are great for holding juice with polyfill and for blue foam, but my 510 isn't good with bluefoam, I haven't tried polyfill out in it yet but think it'll probably need to be slightly more compact compared to the 401 style cart (the juice holder part of this particular cart is a tiny pallet)