I made a 3-point ballast block for my Kioti CK3510H using the frame from an ancient 3-point scoop. I boxed that up, added some rebar, a loop of chain on top, and a hitch receiver, and then mixed up (14) 60# bags of concrete. The finished weight is somewhere between 900# and 1,000#. The block is approximately 40" wide, 23" deep, and 14" tall. The FEL is MUCH happier with the counterweight in place and it helps alot with traction in the recent 24" of wet snow we just got.
It's been mentioned several times in this thread but bears repeating - A CUT with a loader but no backhoe ought to be sold with some kind of ballast or at least a pointed discussion with the buyer. This tractor is strong enough to get you in serious trouble if you're doing any non-trivial loader work without some kind of counterweight in place.