Any recommendations on a replacement battery? I was going to go with
an Optima but they don't list a fitment for the Disco 2.

While getting the head work done the interior light is draining the old
battery in about 10 minutes......

 

I'd double check alternator output and connections. Autozone or related can do that for free.

 

Yes check both the battery and alternator for a draw and load test before you replace the battery. I am running dual Yellow Top's. A 34/78 fits right in and gives you the added ability to use both the top and side mounted terminals.
Mike