I have several t-shirts that fit great everywhere except the length. Sometimes a cami underneath works, but sometimes I don't want the added bulk, especially on a tighter fitting t-shirt (and especially across my chest area where I don't need to add any more bulk). And since exposing my belly is never an option, here's the solution that I came up with - fake layering.
To create your fake layering piece, all you need is an extra t-shirt in your color choice (mine is white) - and it needs to be an old one (or thrift store) that you can cut into. You also want it to be quite snug around your waist - mine was actually a size smaller than I would typically wear - in order to keep it in place when you're wearing it.
I cut 8 inches off the bottom of my t-shirt - you can adjust this if you have a longer/shorter torso.
You'll have a strip now that looks like this:
Slip on so that it sits around your hips and waist and you're done. No exposed belly!
Nobody will ever know that this is what it looks like underneath: