I use a Wal Mart 12V battery in my 40. It is an EVERSTART Group 24, 700 CCA. It fits the battery box BUT, the smaller (-) terminal is close to firewall and (larger (+) terminal is towards the front. Both are on the top of the battery. My original firewall battery hold down has been modified with a spacer to keep battery snug Go to Wal Mart and check them out. Lower CCA batteries are cheaper. WalMart gives a guarantee too.
https://www.walmart.com/ip/EverStart...0-CCA/28275656
I think the manufacturers change the case size of their batteries once in a while so take needed measurements with you. Plus, top perimeter measurement may differ from bottom where it fits in the tray. Be sure to verify your measurements.
As you can see in the photos, I made a copper ring to fit the NEG BAT terminal so that the battery cable would fit without ruining it. I have been told that the SHUT OFF clamp shown should be on the NEGATIVE terminal but, it's been working fine on the POSITIVE terminal for 30 years. I kept using the original 6V cables as they are nice and thick.
Hope this helps you. Oh, Home Depot also sells batteries now.