Blue foam
mod. It is adapted from a Marineland product for filtering aquariums. Use the search function for BFM
mod, or blue foam. You just cut a trapezoidal piece about 15 to 18mm wide and 12mm long, and taper it toward the bottom. Fill your cart to just below the top by 2mm or so, insert the blue foam plug narrow end first, until it is just below the lip of the cart, and add a couple of more drops to wet the top of the foam.
The Highping mod is also popular, and requires no extra purchase. Just cut 1/3 off of the polyfill that comes with the cart, and use only that piece. Roll it slightly to make it somewhat cone shaped, and insert with the broad end at the top, after filling the cart most of the way. And top off.