OK, you do have to do the transitions from rectangle to round. If you’re going to use the stock reed plate w/ Boyensens, the block doesn’t need to be too thick like if you used a 4-petal pyramid cage setup. Probably about 1" thick would do. All you have to do is to be able to have about 5-6 threads to bolt up the manifold.
You can buy a tap at Home Depot that will have the proper drill size on it. You should stick to the usual 6mm Honda bolts to keep it uniform with the rest of the motor.
Start by laying out a block for outside diameter using the reed gasket and cut it out. Transfer the reed bolt holes and drill them a little bit smaller, and then you can tweek them with your Dremel if one is off a bit. After that, strike some lines diagonally from center to center of the bolt holes (X pattern) on the plate and this will give you the center. Drill out the maximum size of the manifold hole, about 1" from the reed side. Then lay out the manifold side with the gasket and drill & tap your holes.
Then you finesse the rectangle side to the round hole with a Dremel. Try to keep it tight and smaller, than larger. The little motors need all the vacuum they can get to keep a consistent "signal" across the venture. It makes jetting a lot easier than having a big, hogged out intake that only runs at WOT and idle. You will find that the transition comes pretty natural if you go slowly and take it easy. You want to keep the transition angle about 30* or so and it will be steeper along the shorter sides than the long one. Look at the stock manifold and try to match it in basic flow design.
Get a nice 6-8 (course), 3/16" or