Bought an older Anvil ATA case on Ebay and only now discovered extensive, but fine, cracked plastic on one side. I got a partial refund from the seller, and want to see whether there's a go-to shellac/adhesive/coating that can get this guy not only serviceable, but still looking semi-professional. Here's some pics.
Looks like a stress cracks. Without replacement of that panel, attempted repairs will likely make it worse.
It won't look pretty, but you could patch those over with the same fiberglass & epoxy they use for fixing Corvettes & surfboards. It's just a fiberglass patch tape that you glue on with epoxy. As mentioned, it won't be pretty, but it will save the structure of it from cracking further: Rock & Roll over it with some stickers when it's dry: