My Grille Block with Fabric Cover

[winkosmosis]

I think the lycra is a lot better idea than what people usually use. Not only is it easy to cut and fit, but it might let air through once the fans start sucking

[Rokeby]

The custom audio crowd does a lot of one-off speaker enclosures using
fabric and probably polyester resin.

Watching a typical video can be instructive. Here's an example:
(Warning; Turn down your volume.)

How To Fiberglass Sub Enclosure Box For Car Audio Speakers / Amp DIY

If you poke around the web using search terms along the lines of "diy
car speaker enclosure" you may find something more descriptive.

Hope this helps.