They make adult sized onesies or snap crotch shirts like toddlers or babies wear over their diapers.
You can wear a onesie that snaps in the crotch which won't ride up as it covers the diaper and then you can wear a regular shirt over the onesie.
I have a onesie that I wear and it conceals the diaper.
Also maybe try wearing a slightly longer shirt so that the diaper stays concealed.
But onesies are a great option for adults, teens and kids who wear diapers and the diaper won't show as the shirt cannot ride up.
Here's a link to an adult Onesie also known as a bodysuit like toddlers or babies wear but for adults.