How do you make a f/g sound as good

Hello All, long time reader, first time poster

I have been experimenting with creating fibreglass boxes, and i need some advice. I hade a 10'' sealed box, there is hardly any flexing on the walls but the wooden version of the exact same box still gives better bass. I just threw some quite dense cloth inside, and it did improve the sound a lot.

My question is what can be put inside a fibreglass box to make it sound as good as a wooden one without adding too many layers of f/g which makes it heavier and then you may as well use wood? I have heard of foam, cant remember the name but it is kind of a egg shaped on the surface, would that work well too?

Tks in advance.

There is virtually no flexing on the walls and yep, box is pretty much to the standards. Going to do more testing, and will keep everyone posted.

Thanks for the advice

arnhem