How can I extrude all faces of an open model as one?

in this example, I do need to clone the open poly surface, the parch the holes with loft tool

is there any functions that could select all of those faces, and extrude them as if it were one?

Another approach is to:

1.) Create a rectangle
2.) Offset it. or make a second rectangle inside the first
4.) Run ExtrudeCrv and set Solid=Yes. Select the two rectangles.

– Dale

I can’t quite remember the name of the function, but it worked something like this, selecting all the faces, then extruding (without using subds)

OffsetSrf, you would have to run MergeAllCoplanarFaces after that.
or you can use Shell, but for this you need a closed cube, click on the upper and the lower surface, these will be deleted with this command and the remaining walls will extrude.

And set Solid=Yes in the prompt.