As long as your fabricating a battery tray, I would suggest to make it with slides. I've forgotten who on the Forum built a tray like this but it was well done.
If you're a serious boondocker, there is too much good about 6V GC2 batteries to ignore. They have the same case footprint as grp 24 batteries, just higher in dimension. I would use 4 of them if I had the space. Sam's Club or Costco sells them for about $80/pc. The 6V's are reliable and have thicker plates which results in better longevity and friendlier with deep discharge cycles.
Another thing hard to ignore is solar charging. Once set up right, it is virtually maintenance free and a user doesn't have to run the genset as frequently or at all, depending on application and use. Storage issues for the batteries are also non-existent. A user doesn't have to remove them or use a charger to maintain them, the solar takes care of it.
Best of luck with your new 12V system. There's nothing like having a good supply of power with you.