Combine several objects (Image masking)

[cje]

Hi, 
I'm coming from Corel and trying to combine several objects into one. 

See enclosed image;
Left: I've made 7 lines which I would like to fit inside the frame as shown. Everything outside the frame should be removed. 
Right: In Corel I would solve this by make a "window" and combined objects to make it look like the inner frame (everything else removed).

Moving layer of grouped strokes to window doesn't work properly (either the grouped strokes covers the border and not inside, or the grouped strokes covers the window and the border disappear)

How can I make it look like the right image (w/o the square around the "window") in Affinity Designer?

[Alfred]

1. Position the frame over the group:

2. Give the frame a fill:

3. In the Layers panel, drag the frame and drop it onto the thumbnail of the group:

[cje]

Thank you

[Alfred]

You’re very welcome.