Hi,
First you need to weld your vertices, since now your model is made of
4 elements, so go to Vertex subobject level, select all vertices, click on the square
setting button next to the Weld button and use a very low value like 0.001 and click Ok,
Now go to Border subobject level, use CTRL + A to select all borders(= two borders),
Right click and choose Cap. You can also find the Cap button on the Edit Borders rollout.
https://drive.google.com/file/d/1_hmEhfdpBsMvTZcjuTskdh7IOtfoHIYx/view?usp=sharing
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miledrizk@gmail.com
3DS Max 2021, V-Ray 5.2
3DS Max user since 2003 (3DS Max 7)
V-Ray user since 2008 (V-Ray 1.47)
My YouTube Channel http//www.youtube.com/c/MiledRizk