"I was thinking you could have your camera facing a chair. You walk into the frame nude, carrying the pump and turn around and bend over so we can watch you insert the tube nice and deep and then you sit in the chair.
You say something like "Today I will myself by pumping air into my belly until it is tight and ready to burst, then I have a surprise for you. No matter how much it hurts, I will keep pumping until there is no more room. Ready to watch my belly swell up?"
You then start squeezing the pump. Take your time, no rush. The slower you go, the more you can take.
Describe ...

"I was thinking you could have your camera facing a chair. You walk into the frame nude, carrying the pump and turn around and bend over so we can watch you insert the tube nice and deep and then you sit in the chair.
You say something like "Today I will myself by pumping air into my belly until it is tight and ready to burst, then I have a surprise for you. No matter how much it hurts, I will keep pumping until there is no more room. Ready to watch my belly swell up?"
You then start squeezing the pump. Take your time, no rush. The slower you go, the more you can take.
Describe the feelings as you fill up, the pressure, talk about it swelling.
Describe any cramps you feel, but continue pumping.
Press in on different spots of your belly with your free hand while you pump with the other. Also playing with and fingering your bellybutton is very hot.
As you get closer to your limit, talk about how you feel tight and bloated and you might burst soon.
Keep pumping until you're at your limit, and tell us there is no more room left.
Ask "Are you ready for your surprise? I want you to touch yourself. I'm going to squeeze my swollen belly until it bursts for you. I want you to watch and don't cum until my belt explodes okay?"
Then drop the pump and put your hands on each side of your belly. Start slowly squeezing in. Moan a lot, act like it's cramping. As you squeeze harder, say things like "Just a little more!" or "My belly is going to burst!" then moan/scream one loud time as you squeeze hard. Then the screen can go black and play your wet popping sound."

This is a custom clip based on the wonderful idea of I.S., thank you so much!