Materials list:
2 - 2x3" 8 footers
3 - 1x4" 8 footers

1 - 1 pound box 2 1/2 inch wood screws
1 - 1 pound box 1 5/8 inch wood screws

Small drill bit for pre-drilling 1x4 materials (otherwise the ends will split).

All the dimensions are on the illustration. They are to the outer edges of the box.  Interior angled braces you'll need to measure, if you get a little out of square, they will vary 1/4" or so.

Be sure you use a square to assemble the parts, an out of square box is not good.  

Assemble the outside main frames first. Stand the motor inside the box with the cases resting on the edges of the bottom 2x3's about half-way onto the boards. Then start adding bracing. A second set of hands is good, but I managed by myself.

Some of the interior bracing around the cylinder will be eye-balled to get the tightest fit to hold the motor in place.  I suggest taking the valve covers off and the bolt that holds the decompression limiter so you get a solid straight brace across the heads. Dry fit the angled braces for the best fit, draw a pencil line so you can put them back, then pre-drill your screw holes. Run the screws about 3/4 the way in then put the board in place then screw them into the braces.

Use 2 1/2 inch screws everywhere you are screwing through a 2x3.  For the 1x4 bracing you can use 1 5/8" screws.  

I had to cut some random blocking in a couple places to wedge the motor tight. You'll find them. (You'll need a 1/2 inch shim under the clutch side case where it rests on the rail to level the engine.)

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