Seems you need a set of instructions to help you out. Don't need a new firewall. Here's a few of pics that might lead you in a direction that easily meets strength and rigidity needs. A little larger engine though. The "stand offs" are over 3" deep. The method has been used on a great many planes over the years, most of them much larger than what you are dealing with. You do not need to go to great expense for machined metal parts or endure a bunch of trial fitting of parts to construct a secondary firewall.

The picture of the cowl/prop hub is for separation references. The picture of the engine is to assure you the method will function well for what I consider "small" engines since it handles much larger ones so well.

There's always a choice between working smart and working hard.