A long, long time ago, when I actually flew models, we would install vacuform or metal formed cowling in the following manner. Once the firewall was ready, we would install a series of 1/4" or larger square blocks of wood around the perimeter of the firewall.They were inset from the edge to allow the cowl to slip over them and go clear to the firewall. Once the set back had been determined, the blocks were filed and sanded until they fit closely inside the cowl when it was up against the firewall and the cowling was secured in place with very small wood screws. If I were building a static model, and using a vacuformed cowl, I would probably secure it in place with Super Glue. Any way that's how we used to do it.
BTW, Who is the manufacturer of your Ju 87?