Extension tubes don't do much for longer focal length lenses. This is because they add only a small percentage to the lens' already existing focal length. And the 75-300mm f/4-5.6 is such a stinker of a lens to begin with anything you do to magnify it further is just going to magnify its flaws.
What you might want to do is use extension tubes on a shorter lens, where the focal length can be increased by a greater factor than the minimum focus distance. Even one of the more recent 18-55mm kit lenses will work for this purpose, just avoid the early non-IS versions (The non-IS III is also good, but so rare and was only sold in kits in the UK so you'll likely never see one). But the best solution would be to use a true Macro lens. Cheaper extension tubes make it difficult to control/change the aperture from any setting other than wide open.
The Canon EF-S 60mm f/2.8 Macro lens can be had new for less than $500 (US). The EF 100mm f/2.8 Macro lens sells for about $600 new.
In either case it is going to be difficult to fill the frame with pieces of the size you mention in your question and have the depth of field you desire unless you use a narrow aperture that requires plenty of bright, even lighting. Controlling the aperture will be much easier with a true Macro lens than with non-automatic extension tubes.