Hi, I built a terrapin pond somewhat similar to yours about 5 years ago and have had very good breeding results. I too used the landscape timbersstacked up with an overhanging lip to prevent escapes on top. My entire enclosure measures 8' x 12'. The pond itself is 8' x 8' and the sandy area is 8' x 4'. The water depth is a little over 2 feet deep. If you look on www.geocities.com/dterrapin there is a link to a great yahoo e-group devoted exclusively to diamondback terrapins. If you join that e-group, I have a photo of my terrapin pond posted in the photos section in a folder titled "Jason's".
I think you will get some breeding results, it is just a matter of time. There will likely be an acclimation period that may vary from a few days to a couple of years. They may need to stay in the pond a year and overwinter outdoors naturally to trigger a good breeding response. The changes in daylight, temperature and so forth that come with the seasons really get the terrapins on a consistent breeding cycle once they have been in the same pond for awhile.
I think you have a great set-up and I would just be a little patient and before you know it you will have hatchling terrapins all over the place.
I just wanted to ask what state you live in just to try and approximate what your seasonal temperatures are. If you are up north it may be wise to deepen one side of the pond for hibernation purposes. Also, it may be a good idea to partially shade the water in the hottest parts of the summer.
,Jason