Here is one way to build your own. I used a center pivot from a garage door, purchased from a local hardware store
The square rod from the locking t-handle inserts into this pivot. I connected this pivot to door sash pins by stranded wire cable:
The door sash pins can be purchased online from McMaster-Carr or from several other vendors.
http://www.mcmaster.com/ look in window hardware, page 2700 of the onlne catalog. They are listed as Window Spring Bolts.
These bolts insert into striker plates that are inserted into the side walls. The strikers were make by welding a small piece of 3/8" ID tubing onto a fender washer.
This set-up worked to allow for locking of the hatch, but it did not apply enough force to fully seat the weatherstripping. Therefore, we added some overcenter latches to the bottom corners of the hatch lid to pull the hatch tight.
These chrome beauties were purchased from West Marine
http://www.westmarine.com, the link is from a piece of chain and the webbing was sewn to get the exact length (by trial and error) we needed for proper function.
I hope this helps.