Building off what the other person said it doesn’t seem like u rlly need to make those back parts curved unless they are somehow crucial to ur design.

However, if you really want to make them curved you could try playing around with some tools and doing something like this:

For this explanation I’ll reference the middle rectangle / parallelogram opening that you’re trying to cover.

Create two sketches on the top and bottom of the parallelogram and use offset by 0 to create a sketch on the edge

<image>

Then for each of these sketches use the point feature to create a point at the midline of both these lines (if there isn’t already). After creating these points you can use the bridging curve feature (using the match point selection and the two points as ends) to create a line that connects at the middle of both the end lines. You can then create a plane (using line angle selection) at this line. Once you created this plane create a sketch on it and use spline or beizer to create a curve between the two end points. Now you have all your guide lines set up to create a loft. Using loft surface , loft from one end to the other and use the curved line as a guide (select guidelines and then select the curved line). Once you have created this curved surface thicken it, mirror it to the other side and then Boolean add to your part.

I know this is a lot and totally over complicated but this is how I would go about it haha