Nice thing is that a boogie board doesn't complain the way a person does when something doesn't fit quite right!
I always suggest making a prototype, old sheets/thrift store sheets are good for such things. You'll be able to wing it just fine, but use big seams (not our quilter seams), a good sturdy needle and thread. When making covers the typical mistake is to not allow enough depth for the item. One way to handle that is to draw an outline a couple inches around the object for top and bottom, and then a long side insert however deep you need all the way around. If looks aren't so important, you can even just zig zag around the outer edges, completely sealing the "bottom" and the leaving the opening for the zipper up top. (Of course, it would probably be easier to insert the zipper first and then attach the top to the bottom.)
Check out some videos on large zipper applications or things like this:
https://abeautifulmess.com/2018/01/h...eping-bag.html
And then there are other ways you can make a nicer bag with inner seams, again, check out some bag making videos for hints on different ways to do corners.