I'm unsure what specifically you're referring to so I'm going to give you a few options on how I would create this top generally; This is a classic example of cartoons don't have to deal with gravity/physics so be prepared to make some concessions on "Screen accuracy" for the payoff of not flashing anyone at the convention.

The skull generally:
In every scenario the skull would be a non-fabric, relatively rigid piece. I'd most likely make it out of EVA foam but if you can find a pre-made mask or something - given it is Halloween that is an option as well. There is the option to kind of make it like a stuffed animal, but that seems harder to my brain than the first option.

A couple of options here for construction so bare with me:

Option1 (which would be my personal choice, based on my body):
I'd personally opt for just creating a standard tube top (slash strapless dress bodice) and extending the bottom to fit the design appropriately. This way you have to forego the sideboob, but there's minimal to no risk of flashing people accidentally.

I attach the skull with both some big snaps on the back of it, and some flipper bones to shove down into the top corset (if you don't know what these are Minji Lee talks about them in a couple of her YT videos - just google search flipper bones sewing and I think it should come up) . Small release clips like you can find on bookbag straps, or paracord bracelets would also work, as long as your red tube top has some flipper bones as well.

Option 2: For if you want the sideboob

If you absolutely want the side boob, that means making the red top either totally an underbust situation (meaning the skull mask contraption is your only coverage for your chest, in which case I'd PERMINATELY attach to it a triangle bikini top, and make the top straps clear, and tuck the back strap under the red top) If you are blessed to be able to go braless- you could attach a clear straps to hold the mask up. and cross them into the back of the red top like a regular strapless bra/bra where you can convert/change the straps, and slap on some pasties just in case. I'd also attach the mask to the top edge of the red top here, either permanently, or with pin backs. Depending on your body this is also going to mean you need to very strategically shape/build said skull mask piece.

Option 3: Max coverage option:

The third option is to make the red piece underbust, sew it on to a tight/stretchy purple top, and make that whole piece just one shirt. This is the max/full coverage option. If I did this I'd just use pin back or snaps to slap the mask on.

This character is really cute! good luck with the cosplay <3