While diagnosing the faults my Land Rover was reporting, two dealerships told me that my OEM battery was weak and should be replaced. The truck is now 3 years old and it seemed a good time to upgrade to a high performance battery. Years ago I bought an Optima Red Top for my previous vehicle, but I was surprised to find a lot of negative reviews about them. Apparently the quality of Optima batteries has dropped. The online forums show many owners reporting them failing after just a few years. I saw several posts by Land Rover owners who have been installing Odyssey batteries and are very happy with them.
I bought the Odyssey 34R-PC1500 (aka PC1500RT). This is bigger than the BCI group 94R OEM battery. I could have bought the PC1220, which is 94R, but chose to get the bigger 1500. I bought it online from West Coast Batteries (the links above) and it was delivered in just a couple days.
Since the battery has a slightly different profile, I had to make a new hold-down bracket. Skipping the methods that failed, I'll just describe the final solution. I bought a piece of aluminum (3ft x 1 inch x 1/8 inch) from OSH and headed to a friend's house where he has lots of tools. We cut it to size, bent it, and drilled holes, so I could use the OEM bolts (which seem to be metric M6 size). The photos show the final installation. Since the Odyssey is taller than the OEM battery, it barely fits under the plastic cover (not shown in the photos). The end of the bolt on the positive terminal touches the inside of the plastic cover.
Now I'm ready to go adventuring off road again. Good thing, since I'm headed to southern Utah and northern Arizona in a few days.