I made boards with db15 connector and thermally conductive 2-part epoxy. We did a lot of boards, had bigger bottles:
https://www.digikey.com/product-detail/en/mg-chemicals/832TC-450ML/473-1089-ND/655660
Needed 2 hour bake in a low temp oven to cure it, or wait a really long time. Maybe we hadn't stirred both parts well enough before mixing together and pouring into a mold with the card. We were doing 12 at a time in square muffin pans.
Selected all high temperature range parts, including the 328, rated to 105C or higher
https://www.digikey.com/products/en/integrated-circuits-ics/embedded-microcontrollers/685?k=atmega328&k=&pkeyword=atmega328&pv252=774&FV=ffe002ad&mnonly=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25
Had to seal around the bottom of the connector with bead of silicon before potting to keep the potting from seeping up into the bottom of the connector. Learned that the hard way.
Can go higher temp on the 328, may have to wait longer to get them
https://www.digikey.com/product-detail/en/microchip-technology/ATMEGA328P-15MZ/1611-ATMEGA328P-15MZCT-ND/6832161