Ah.... I see you're using the lab potting technique we used on cables in the 1980's. We had forms for some of them but others, especially for pre-production quantities, we did free-form with masking tape - worked great. You really can't beat submersion potting for anything in a hostile environment (and your car, at any speed, in rain, qualifies as a hostile environment).
Snow's just starting here at 10am. Another 6" due here in the snow belt of central Mass. 
We're sitting somewhere around 2'+ on the level after compaction. I'll be working inside where it's warm. Just checked to see if the velcro adhesive on my heater controller stuck after I heated it up before applying.
It's still sitting there, stuck under the radio.
