After listening to your opinions and thinking hard, I decided to buy this because I think the most similar thing to denim insulation sold in Korea is insulation made from recycled clothing.
The thickness is 0.4" in inches and it is 35.5"X393.7" 19.22$.
I'll have a lot left over after I use it, but they don't sell it in a smaller size than this, so I decided to go with this.
Thanks for your help and answers.
The speakers are used and I spoke with the seller and will pick them up next weekend.
It will probably take me a month to buy the parts and modify them.
I'll leave a review after I do.
View attachment 361791