Here's what I did; At the front of the fuse, where the base cowl ring mounts, I cut about a 5/16"- 3/8" smaller diameter circle from the fire wall. Leave the fire wall original size where the four screw mounts are located. Notch out the sheeting in these undercut areas towards the rear of the fuse about 2 1/8- 2 1/4". Resheet the fuse in these areas. You'll end up with slots that taper and when finished aren't that bad looking. Seal off completely front of cowl. Cut a 1" slot from crankshaft, in front of carb, to edge of cowling in the front sealing material. Should give you 3 to 1 ratio. Worked in mine with a OS61 FS. You might have to fine tune for your application. Hope this didn't confuse you.