94 Silverado C1500 Air Distributor

I’m fixing up my late grandpa’s 1994 Chevy Silverado c1500. The air inlet actuator plays the drums every time you turn the key on or off. So I’ve pulled the entire dash and distribution box to clean out a squirrel nest and swap the Evap core while it’s out and I’m throwing a new to me dash core that’s in way better shape. My issue is that the box has upper and lower half’s that are sealed with an with some kind of adhesive sealant in addition to metal screws on the corners and I don’t know what’s a good replacement. It’s in the hvac so I don’t want fumes blowing into the cabin. I want it semi permanent so I can service it in another 25 years. I want it non hardening so it doesn’t flake but non of the big three auto store have suggestions. I’m also curious what sealant are they using around the seat anchor bolts that’s still moldable to this day. It a waterproofing rubber compound it seems. Any ideas? Thanks