Sealing Hatches with gasket material

[Cheechako]

If you want to just repair what you have, I would be tempted to just use black silicone to fill the gap. Maybe cover with wax paper until it hardens... will wax paper release silicone like other things? Not a big user of silicone but it is tenacious and very flexible.

[blueazimuth]

A photo can speak a thousand words, and this one does.
Both the length and angle are incorrect. Get both of those right and the adhesive becomes less critical. (Contact cement has its applications, but this is not one).
However, getting both the length and the angle correct, on a corner, and with one cut, is a lot easier said than done. Somebody offered the suggestion of a side join - IMHO that's the way to go - you'll find the join easier, and a lot less frustrating...