Yeah a pull-up bar and a very basic set of free hand weights mixed in with push-ups and somewhere to run will go a LONG way when used correctly.

However it's your choice, so I'll actually answer the question you're asking now
I don't know if this idea is any good or just crap, but maybe just those lead bars made for melting down and weighting other things... you would definitely want to cover them very well or something though. I just thought it fit the weight and shape criteria.