How to secure a baby onesie into shadow box frame?

Could you pin it to the back board?

I pin it to the back mat board in place. Hot glue sometimes come loose after awhile.

Double sided tape n glue gun

I would pin it to the backboard with t pins. T pins are like straight pins but they form a t shape. If you have to add a thin piece of styrofoam into the back of your frame that will give you a bit more to pin your onsie into. You could put some decorative paper or pretty material over the styrofoam to hide it from view. I would not use hot glue as you would never be able to get the glue out of the onsie if you wanted to remove it and save it as a keepsake.

use stapler/staples on inside of onsie so you don't see them if you use pins cut them off with wire cutters or needle nose pliers

Double sided tape n glue gun Dude, that's brilliant! Hahaha After reading the thread, I thought about this question, but as soon as I read your comment, I was enlightened, LOL In fact, I think that this is not necessary!! Now I bought cool Pokemon pajama on an online service https://www.kigurumi.ca/ and I liked how convenient it is. Your version sounds so logical, but on the other hand it is so pointless if you look at it from the point of view of practicality and meaning! After reading this thread, I understood several aspects of this question, no matter how ridiculous and paradoxical it may sound.

Use cute push pins to attach clothing items.