Method 3
somewhat follows the line of reasoning of method 2
create rectangle 900x50 and pushpull it 100mm, make it a group "middle"
create 45deg wedges either end, groups "bottom mitre", "top mitre"
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make it a group called "side" and create the relationship / formulas, again I used parent as I intend to copy and make unique sides
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now hide the joining geometry and group, this then in formation should be similar to the side in method 2, but mitered rather than over lap
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do the rotate @ 500mm as before, copy(control) *3
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change the group instance name in both model info and the attribute dialog
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it is important to confirm that the groups are unique (changing the instance name or position may not create separate definitions (the copies initially share the same definition, that relationship needs to be broken) so pair each one and right click make unique
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now make all the unique sides a component, "frame3"
fill in the relational formulas
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completed example attached