I solved a similar problem on an ME-109 by epoxying hard wood blocks to the firewall and than mating hardwood blocks to them with large screws and blind nuts. This assembly is sanded to fit the inside diamiter of the cowl. You can access the screws when you remove the spinner. I then slide the cowl on make sure it is postioned perfectly and then tack glue the outside blocks to the cowl. You then remove the cowl and epoxy the blocks in permanently. No exposed screws. You will need a long screwdriver, but thats easy.

Regards,

Dave