I solved the test but after that I can't find the exit, there's a broken catwalk but I don't see portal surfaces

Not bad but very easy.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=402740732

To complete the chamber you need to - as you said - fast portal under the cube to cause the door to open and light bridge to appear. There is an invisible wall in the current version over the door when the door is closed so you can't land on the frame as that was rather inconsistant. Thanks for the comment!

By the way, there is only really that chamber in the map as I'm not the best at making good puzzles but trying to get better. The main attraction are the sights outside the test.

How to exit the first chamber ? Land on top of door frame or fast portal hit under the cube? Nice work - thanks for creating.