Bryan,
I purchased my battery box direct from EarthX. It is incredibly light (only 140grams) and ready for install. I knew it would save a lot of weight and time over fabricating my own. I created the bracket to hold relay out of 6061. If you are going with an EarthX, my opinion would be to pick up the box now so you can put it into position. The battery is good just sitting on the shelf, so I would buy the two together. They often give discounts if you mention Oshkosh. I think I got 10% my total order.
Also, you can find really nice lithium chargers for cheap which has the kicker function built in if you run the battery dead. Mine is a Schumacher Automatic. With EarthX, you just need to kick it to get it back, which has saved me about 8 times already. I really like the safety feature EarthX has built in.
Also, I would recommend you mark out your throttle cable, mixture cable and wiring pass-thru with minimal bends. Be sure to account for the fuel tank inside the firewall.
I picked up my relay from a Google online search as it's much cheaper than an aircraft store.
The model I am using is a White-Rodgers 70-111224.
Zoro is local and has it $28 shipped. If you search, you can find it for as little as $18 on eBay.
https://www.zoro.com/white-rodgers-dc-p ... lsrc=aw.dshttp://www.ebay.com/itm/WHITE-RODGERS-7 ... SwMgdX0t-3