Hawk,
If I can help you in any way, I will.
The dimensions Zuma gave you will work fine and should give you a high pitch of a young turkey.
My first question to you is what kind of tools do you have to work with? You need a table saw, drill press and a belt sander to get started.
Neil Cost, the greatest of all of the custom callmakers said "Poplar. mahogany and cedar are the best woods for a box call", too That I would add American Chestnut as another good choice.
Like JW's pal, I made my first box call from a 2x4.
There are 2 basic styles you can choose to make, the sawn and glued style where the pieces are cut out and then glued together and the hewn out style, where the sound chamber is hewn out of a single block of wood.
The sawn and glued model is the easiest to make, the hewn out model allows you more control over the quality of the sound the call will make.
Here are some dim's for a s&g call,
Paddle-3/8 x 1 3/4 x 10, put radius on bottom of paddle with belt sander. Place sander on table upside down and try to get radius as even as possible.
Sides-1/8 x 1 3/4 x 6 1/2, Radius top edge 1 5/8 at the middle and 1 3/8 on the ends.
Base 3/8 x 1 3/4 x 6 1/2.
End Blocks- 1 x 1 1/4 x 1 3/8, saw angle on block to make it 1 1/4 at the top and 7/8 at the bottom.
At this point glue everything up, drill some holes for the screw and spring and give it a try.
You can adjust the call by moving the screw in or out to find the best tone your call will make.
If your call is to low in pitch try making another call with slightly thicker side walls, if your call is to high in pitch, use a good pocket knife like a cabinet scraper and scrape the inside of the side walls to make them a bit thinner.
I hope this helps, the hewn out type is more complicated but worth the effort, if you want to make this style let me know and I'll get you some dim/s.
Kevin