You have a couple of options to replace the cover. The cheapest way is using the aftermarket kit by Sierra for $12 or so.
I've included the OEM part numbers as well if you'd prefer to go that route.
Aftermarket: Sierra 18-7044
OEM:
0341071 Solenoid Cover
0341297 Cover Gasket
0175158 Maintenance Valve Assembly (Usually Includes 0126341 Maintenace Valve Cap / 0331365 Valve O Ring)
Thanks guys super helpful! Got parts ordered and on the way
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