I have the older cousin to yours a L4600DT.
I use my carryall as my ballast, it weighs in around 750#'s to 800#'s. I use the carryall itself, 5 80# buckets of concrete, and 150#'s of weights secured under the carryall.
This seems to work well for me, with bucket full off dirt but kept low when moving it seems to handle well.
I like the carryall as ballast as it can serve a number of uses when not needed for ballast. I have $200 in the carryall and the concrete and broken bricks and blocks less than $20.00. If I need more ballast I can buy a 80# bag of concrete grab a 5 gallon bucket from the barn and presto more ballast.
I remove the buckets which have lids and stack until needed next time.
If you like my idea you can search my post for carryall build and gets more ideas details.
Good luck.
Jay