flight test ballast ideas?

Can anyone give me some ideas for temporary, but securable weight
ballasts? The situation is we're preparing for FAA required test
flights on our FIRST production aircraft. Weight will most likely need
to be positioned in the right seat area, and/or in the limited baggage
area behind the two seats. Wing bladders are not an option, nor is
anything requiring a permanent mount. A non-U.S. built model of this
aircraft has already been certified for U.S. flight. This is the first
built here; but is not a prototype or "test model". This aircraft will
be sold to a private buyer once it passes flight tests. This is the
reason for the ballasts needing to be "non-invasive". I was thinking
of something that could be bolted to the right seat mounts and/or
strap secured in the baggage compartment.

Any suggestions or sources?

Thanks,
DocS