From my very limited experience: "Easiest" way to do it would be to find a pre-made vector file of the depth contours for the lake you want. It might exist on someplace like Etsy.
The distant runner-up is to take one of the available sources of bathymetric data, take an image capture(s), import into a graphics program and draw or trace the depth contours you want. IME, auto-tracing doesn't work too well for this - lots of extraneous artifacts leaving you with confused and broken vectors. It's a lot of computer-time.
Here's one I did. I turned bathy data from an even shallower lake..
View attachment 216269
..into five contours. This is on a ~3.5" tall Christmas ornament. The contours here - 10, 28, 34, 44 (I think) - are of irregular depth and the contours themselves are cut proportionally deep.
This is obviously done on a CNC, but exact same principles apply - ignoring the 3D carve
View attachment 216270
-Mark