I have wanted to put a larger 2nd battery under the passenger seat for a while now, tired of having my house battery floating around the back when I'm camping.
Had a dig around here and on the net, could only find solutions that either required major surgery to the OEM cables or used a smaller battery than I wanted. I was aiming at 100Ahr or more.

Had a measure up, a bit of guess work and this is what I came up with


Haven't got the tie down right, but I'll fix that up in a few weeks.

Here are the pieces.


Test fitted, well after about 6 measures and cuts.

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Large piece is raised up 48mm to house the cranking battery, this lets me slide it over about 60mm
Small piece is raised up 21mm, which brings it up level to the inside bit of the floor. (well 23mm would have brought it level.

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Here they are welded up. I've included some dimensions but my advice is not to trust them completely, you'd have to check the bolt positions any way.
What is not shown is I added some cutouts to the small verticle section to get some straps thru to tie down the cranking battery.
I've also added stops back and front on both batteries, to stop the buggers from sliding foward or back

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Installation, I've put a 6mm bolt beside the -ve terminal to show the clearance. On the house battery, there's about 10mm from the top of the terminal fittings to the bottom of the lid. Given that it's the negative I'm happy with that. The cranking battery is about 30mm.

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The house battery is a Maxon MXG12-105 deep cycle, it a pretty big battery at 306mm long x 169mm wide and 214mm high (thats from the bottom to the top of the terminal, no fittings).
There are no changes to the original battery feeds, but it is pretty tight to get the Positive on, once it's on it sits nicely and there is a bit of movement in it.

If I were doing it again, I'd get the plate folded so it's one piece and I'd use 50x50 RHS on it's side for the larger spacer and probably cut the top of a bit of 25x25 for the smaller spacer and run it as a U.

Cheers Glen