Good thinking!
However, I worry how you will be able to handle the stuff if you weld it together... might want to create a platform and use numerous 1 ft chunks to get the needed weight. 130lbs each would be enough to handle, much less several of them together.... however, I bet if you explain what you are trying to do to your maintence folks, they will have a bunch of stuff that you can use... maybe angle iron, etc... I assume you can cut metal... go for it!!!
I have numerous chunks of iron that I use.. on various implements, as needed to provide weight... the flexibility is nice.
Again, think about how you can create a carryall on the back.. you won't regret it... and it will make a platform for your chunks of iron. As you expand your activities, you will discover that you will nearly always need extra weight on the rear.... that FEL is so handy that I never know what I'm going to grab with it and need to carry tools, etc everywhere as well..... also, figure out a way to carry a bunch of chains... maybe the maintence people have some busted ones you can pick up as well...
I assume you have chain hooks on your FEL? If not... they are very very handy.. can't do without them!