Part 3.
Final.

Frame.

1 Select the center lines and from the front or right view move them to the desired distance.

2 From the '3D' view, use the 'Lines/Line' command to draw lines as in figure "III_1".

Surfaces.

1 Select the triangles and apply the "Planar" command to them
2 Select 4 lines as in the "III_4" image and apply the "Network" command to them.
3 Using the "Sweep" command build a surface like in the image "III_5".
4 Select the 2 surfaces, image "III_6", and apply the command "Boolean/Merge" to them.
5 Remove the unnecessary parts.
6 Build the remaining surfaces with the command "Network" as in image "III_7, III_7_2".
7 Combine the constructed surfaces with the "Boolean/Union" command.
8 Unblock the oval and apply the "Plamar" command to it.
9 Select the resulting surface and apply the "Boolean/Diff" command, select the constructed surfaces and complete the command.
10 Hide the plane of the oval.
11 Select the oval and apply the "Extrude" command to the desired height.
12 Remove the top part.
13 Return the hidden part and use the "Boolean/Union" command to merge the desired surfaces.

https://disk.yandex.ru/d/9I0HXeZKOr5h9w

This is only one of the options, and not the best, try and feel free to experiment.

That's all.