This maybe relevant to all S Chassis so wasn't sure where to post.
I was browsing for decent quality battery tray, particularly OEM trays to relocate mine and was playing around with idea's how and where to mount it. I did consider above fuel tank and came across Skyline trays. Didn't realise most R33's had boot mounted battery's and guess what? above the fuel tank too!
Never seen anyone do it this way before so thought I'd buy one and hopefully make it fit.
Guess what else? I found 2 predrilled holes on my S15 that match up, Result! That's half the tray mounting sorted.
Both holes are covered by stickers, one hidden behind sound deadening.
Do S13 and S14 have them too? Have a look and say so, may help others.
To cut a long story short
This is the actual tray I bought:
The pre drilled holes, Pic is after I riv-nutted one of them, M8 iirc but I think M10 riv nut might be better fit:
And the finished result, I'v still to buy a big battery:
Now for the long version:
Seems to be 2 trays, I bought the black version from JDM Garage and painted it Gray to confuse people
Apparently cars with the cold climate option and most 4 doors had the bigger battery, not my pic:
My S15 came stock with what looks like mx5 mk1 sized battery, tall thin and fooking expensive.
This is a MX5 mk2 battery, lower, wider and much cheaper, this is a bit too big for engine bay with intercooler pipe but can be forced in with some butchery, this was also dying but as you can see lots of space for bigger battery.
I think 075 would be good as it's the same height as mx5 mk 2 battery (rear speaker interference).
The recess is where battery clamp will go:
I'l soon replace the circuit breaker with 100 amp, I never once blew a 140amp fuse with my old relocated S14a:
The large 12v thread stud near the zip tie was fairly exposed and being before the circuit breaker wasn't safe in my opinion, so rubber cap over the stud and cut the clear plastic shield to size, I pulled all the fuses to kill the connectors:
Parcel shelf had a small existing hole I Riv nutted (M4 iirc), the metal plate provides some extra projectile protection. R33's had a metal firewall cover, I might replace the plate with something beefier to be safe, remove carpet padding where tray sits, tray will sit flat despite large hump above fuel tank:
Used MX5 mk 1 earth cable, remove paint when mounting of course, as you can see not much speaker clearance. For deeper speakers, middle speaker hole could maybe help battery is budged over:
To keep it small I used a 2 prong box from vehicle wiring products to merge positives connections, mounted near BMC. Cut the 2 plastic connectors of, strip, crimp and good quality 3:1 glue heat shrink. For old main positive terminal, cut off loop and enlarge hole:
I used the redundant neg as another earth mounting. Original fuse box 12v+ nut can be used for charging/jump start.
A acid neutraliser mat and/or plastic tray is probably a good idea, leaking battery cause horror movie like epidemics.
Something I overlooked, find a vent hole on the battery, and er vent it.