Here is a pic of the plate I made to surface the air horn. You see the length from the ruler. Width is 100mm and the thickness is 8 mm.

The slots started out as two holes at each end of what is now the slot. They are about 3/8". Just need to visualise where you start moving the air horn and where you finish as you move it back and forth across the plate.
I then drilled a series of holes between the two end ones and then with combination of a fret saw and file cut out the bits in between and filed to give a reasonable surface so the bits hanging through don't catch.

(Whilst this is not a precision ground surface plate, a piece of 100mmx8mm stock is flat enough to achieve a sufficiently flat surface to ensure the two parts can be mated together and seal.)

Then it was just a case of getting a sheet of wet and dry and start rubbing. I may have used some 120 grit with the kero as that really aids the cutting ability of the wet and dry.
As you start you will see where the wet and dry is cutting and where it is not so you will have a visual guide as to how you are going.

When I first surfaced the main body the only thing I could come up with for a surface plate at the time was a piece of plate glass which happened to be a sliding glass door in a cabinet I have in my shed. Worked a treat.

The main body is the easiest one to do.You just have to strip it down so that the various plugs and bits sticking out aren't in the way . When I did mine I was doing some other mods so had actually removed the fuel discharge nozzles as well.Otherwise just taking out the air bleed screws should give you the clearance you need. Once in that state surfacing was a breeze and in 5-10 minutes it was done.

All of this was inexpensive but does take a bit of time. Well worth the effort however.

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