How do I fully define a sketch that was projected from a STEP file in a new Part Studio?

SeeMikeHack

Big picture: I'm trying to design a panel that will fit inside a specific case that I can mount components to.

I obtained STEP files for a Pelican Vault V100C case from Pelican's website and imported them into my workspace. I projected the openings of the top and bottom of the case (they look to be of the same dimensions) onto a sketch in the imported Part Studio, then copied that sketch to another Part Studio so I can create some openings for panel-mounted components and extrude it for manufacturing. The sketches that were projected from the imported STEP files are of course fully-defined, because they're just referenced off the existing part; however, the sketches pasted into the "Body Plate" and "Lid Plate" Part Studios don't have the same reference. Because the dimensions of the case aren't going to change, I'm tempted just to use the Fix constraint for just the projected lines and curves and call it a day. When I projected the curves from the imported STEP file, I did notice I picked up some center points that obviously define radii for the curves, so maybe I could draw some construction circles for those and Distance the center points from the origin, and that might constrain the rest of the sketch enough, but maybe not.

For this specific use case when I'm lifting a sketch straight off an existing model, is it worth it to try to fully-define the sketch without using the Fix constraint? Or is there something I'm missing about how to easily define a sketch like this? Link to the document:

https://cad.onshape.com/documents/6e18a6953a9ef22b109b5779/v/19260a6702f40f81bcafc46c/e/46c022689e19de8c6d2fac37?renderMode=0&uiState=64d0775dffa6846e59c27310

S1mon

I would probably just “use edge” and align things to the part studio from the step import. The derive feature is a good way to transfer that sketch into another part studio where you can start modeling around it. 

You could instead model your part in the context of the imported assembly.

I almost never use the fix constraint in a sketch except as a temporary hack. It doesn’t really capture much design intent.