Adding extra weight to flatcar isn't funny at all. I recently got an old Athearn flatcar which was lacking a weight plate and few parts. However, the model was in good shape and fitting perfectly the layout theme.
After thinking how I would add weight while keeping the underframe detail, I came to the conclusion the easiest way was to simply remove the it where it can't be seen, i.e. behind the fishbelly. I cut the undeframe in three, removing the central portion and glued in place the end sections and bolsters. Stripes of lead weight were attached under the shell. Easy as 1-2-3 and undiscernable for an operation oriented model. The weights can be painted and it wouldn't be a bad idea to glue brake details on top of the lead if they are visible on your prototype.
I plan to convert in such way a few other cheap flatcars suffering from low weight.
By the way, the rolling stock catalogue is progressing at a fast pace and I hope to inaugurate it by the end of August.