These are Install Bay brand Silicone Speaker Baffle Kits sku: IBSBF80
 
These are perfect for a 8" woofer and greatly improve sound quality.
The way these work is that they help seal the front wave of the sound to go through the door panel. If you dont use a product like this you have a huge potential risk that the sound will bounce back off the door panel and cause cancellation in your music. The cancellation will be most noticable in your mid bass.
These also have a back wave foam pad that is peel and stick. You put this on the door skin behind your speaker and it will help absorb some of those reflections as well.When you use this product you will likely notice a night and day difference in the sound quality and midbass response that your speakers have. Once I started using this product I don't like installing speakers without it. It makes that big of a difference!! I buy these in bulk so the product i resell does not come with the fancy cardboard box packaging but will be shipped with all the correct parts for one pair of speakers.

Dont forget to add sound dampener when you do your installation of the speakers to get the best sound possible out of them: 
https://www.franksaudiomotive.com/speaker-install?Collection=Sound%20Deadening