[mio]

Besides the HVAC vent insulation mentioned above, another option is a generic roll of car insulation (i.e., sound deadener and heat barrier), trimming it to size using your original pad as a template, and reusing the insulation clips. I went with the non-adhesive backing variant given that the OEM clips work just fine at keeping it in place.

 

[mio]

Where did you find the stuff you used at?

There are many variants but this is the one that I got several years ago. I also went with this sized roll because I was using it on multiple cars.

 

[mio]

Mio, what did you do to achieve the battery in that orientation?

It's an aftermarket battery tray (I do not recall the brand) and, as you can see, the battery is designed for a Mazda Miata.

 

[mio]

Well done! It looks really good.