A ¼” braided poly rope should work well. Just remember to heat fuse the cut ends! Have fun!

They make special “leashes” for boogie boards that consist of a velcro strap that goes around your bicep / wrist and is looped on to the board at the other end. But Cruiser’s poly rope would probably work just as good and be a lot cheaper. Velcro might be more comfortable though at the arm end. You could even go without a rope if the surf isn’t huge and you aren’t going out far.

I reccomend using the rope @Cruiser suggested, and on the underside you’ll want to tie a big knot, with a large washer to keep the knot from pulling through the foam.