How do I create a turtle like shell with this sketch?

[spaceman21211]

I feel like I have tried everything, but I cannot figure out how to make the radiused back part using the lines as a guide. 

[TheCADWhisperer]

@spaceman21211 

Did you import portions of your sketch?

I don't understand how you sketched these curves in Fusion (and I have been doing CAD for a long time).

Edit: OK, I figured that part out...

This looks like a complex project for a beginner - is this your first project?

[TheCADWhisperer]

@spaceman21211 

Make use of symmetry about the Origin and use dimensions at least on the straight lines.

P for Project of points between sketches and Coincident Constraints to tie together connecting points.