This is the sink as you look straight on.

The bottom side of the sink is not flat/finished but the whole edge is so do I cut the sink base cabinet front to fit with the sink setting on the edge of the front of the cabinet? Or do I need to pull the sink forward as much as I'd like and let the bottom of the sink land wherever? There will be the 2x4 supports inside. I guess I was expecting the sink bottom to be flat and it isn't. You'd have to lay on the floor and look up in order to see that the sink bottom isn't flat.

and the bottom of my sink is just like yours and i've completely forgotten it was like that til you made me bend over to measure! so i wouldn't worry about that.

I will call the piece of mdf that we scribed the "sink surround" because i don't know the proper name...

at first we tried lifting the sink and tracing around it, but it was impossible to be accurate. so what i wound up doing was, with the sink in place, i measured the space between the edge of the sink and the end of where the sink surround should end (to line up with the cabinet doors below)... let's say that was 2.25". then measured the space from the bottom of sink to just above the doors below... let's say 5". Then I cut the piece of mdf the total overall width top and bottom, then marked off those measurements and cut that bit out with a jigsaw. So I was left with a U shaped piece 2.25" on the sides and 5" on the bottom.

i think we did some sanding to get it a really nice fit. I have a teeny bit to caulk on the corner where it turns from side to bottom, but it's barely noticeable now, with caulk and the color matched paint it will be perfect.

now like i said, we used mdf so we could have just done it over if we messed up without being devestated. but i think your cardboard looks pretty good, looks snug to fit.

good luck! i want to see it when you're done!!