Classic Finn wrote:Ok Kerry now I see it.. now what would be the way to get that spar notch
made into the wall?
It came to mind that I could run in with the jig saw a bit at a time but to get the cross cut into it may be more of a challenge unless I chisel it out a bit at a time..

That partial notch is a real good idea instead of going all the way thru..
Classic Finn
If you have room, a router would be best, but you have to make sure to use a template to keep from cutting outside the lines.
A chisel to cut the outline would be a good way to start, then maybe the router to cut the majority of it out and finish with a chisel. If you use a router, be very careful. It can get away from you very easily when using it freehand.
Using a jigsaw would be very dificult, I would thing.
If you have room from the top, then the jigsaw might work to cut the vertical and use a chisel to cut the horizontal and then just chisel it out.
You really do not need to make the partial inset, it will just add a bit a strength. Either way, make sure and predrill the spar for your screws to toenail it in place and try to use long thin screws that will be less likely to split the sidewalls. Also, make sure to calculate the angle to prevent the screws from coming out the side of the sidewalls.
Sure hope this helps.