I have a personal liking for tuna salad, and wanted a new and different way to present it, away from the ubiquitous "Wonder Bread", so was asked to do a potluck lunch for my wife's ward at work, and hit on using tomato as the "carrier" instead of bread, and found the textures worked well together. You want to find "sweet" tomatoes or low acid ones, and don't be afraid to try out the yellow tomatoes, they work very well! Tomato size is about 2.5" in diameter.
Use CloverLeaf flaked white tuna, best if "packed in water"
Best if you can prepare and serve immediately, as tomatoes do poorly in a fridge; if preparing ahead of time, put the stuffing in the fridge and keep the tomatoes in sealed Tupperware on the counter.