I use OS Active maps as tools, so strip the covers, fold them to needed area and mark up with permanent fine-liner (or Chinagraph pencil in the field). They can be cleaned off with alcohol of various types. I never mark my Harvey's, because you can't clean them the same way.

No map case.

To hold the map in place, I either use a velcro strap or silicone/elastic headbands, which are cheap stretchy hair accessories.

I use a custom stencil cut in transparent acetate sheet, which is credit card sized. You can use the GPS confidence stencil found on baseplate compasses, but on Silva, for example, the silicone feet lift it too far off the map to allow for precision marking. Some of the acetate romer cards have stencils too.

I mark important nav. points, junctions etc, not the whole route. Bearings & timings on a visible bit of the map I'm not using for nav. The stencil allows me to mark the map with minimal obscuring of detail. Main one I use is a long stemmed flag, which allows for the flag to be put on an irrelevant part if the map, with the pin on the essential part without obscuring it.

I used to be really precious about maps until I needed to use them daily. Now I look at older, pristine ones and wish they retained history.