Question about installing hard points for mounting

I think I saw an answer for this somewhere in the real long thread but I couldn't find it when I looked for it.
I'm planning on installing things like porch lights, side markers, small indoor propane heater, etc. and want to install plywood hard points in the foam so I can securely mount all these things. I really don't want to have to cut all the way through the foam, I just want to flush mount the plywood.
What's the best way to cut out the shape to the right depth? I thought about using my router but there are 2 problems I can think of. 1) I suck at routing 2) The high speed of the router would burn not cut.
Any other suggestions?



I'm planning on installing things like porch lights, side markers, small indoor propane heater, etc. and want to install plywood hard points in the foam so I can securely mount all these things. I really don't want to have to cut all the way through the foam, I just want to flush mount the plywood.
What's the best way to cut out the shape to the right depth? I thought about using my router but there are 2 problems I can think of. 1) I suck at routing 2) The high speed of the router would burn not cut.
Any other suggestions?