Fitment of small AGM battery to an AW11? PC680 or HC20

Hi all,
 
Currently my AW11 has a standard lead acid car sized battery installed in the frunk where the spare wheel would sit. I am in the process of buying the correct space saver wheel, jack and tools so I need to move the battery. Been driving around without a spare, jack or toolkit for too long. I'm worried I'll get a flat and be stuffed.
 
I'm thinking I will go with a PC680 size battery (either a HC20 Fullriver or PC680 Odyssey)
 
Has anyone mounted one in an AW11 somewhere apart from the engine compartment? If so can I see a pic of where you installed it? and has the smaller battery been working for you?
 
An idea I have is trying to fit it in the boot where the airbox is on an 87/88. I have removed the airbox as its not being used with the 3sgte leaving a space that it might fit in with a tricky bracket or two.
 
Any ideas would be helpful :)
 
Cheers,
Nick 

Ended up mounting the current wet cell battery in the corner of the boot where the airbox was. The trim cover even goes back on so you'd never know it was in there :)
I think the battery is rated at 330cca.
The cars frunk is now back to stock for the first time with the spare mounted and all the trim looking factory.  

Was actually very easy once I started and got into it. I just removed the factory airbox & resonator and utilised the bracket which secures them to the body. Its quite sturdy this particular bracket.
I used a flat piece of carboard and gradually trimmed it until it filled the whole space whilst sitting ontop of the airbox bracketry.
Traced the cardboard cutout onto some 2-3mm alluminium plate and bolted it down onto the airbox bracket once cutout. 
I placed the battery ontop of the alloy plate and marked the battery external dimensions on the plate with a black marker. Removed the plate and attached a small bracket on the side for the battery clamp/rod to attach to.
Cut some thin foam and rubber matting to suit the shape of the plate for the battery to sit on as a shock absorber.
Then put the plate back in, foam & rubber mat down, battery in, battery clamp holding it down and started to figure out the cabling.
I used the factory air intake piping hole which passes through the boot firewall to run the cables into the engine bay. I might seal this black plastic pipe with something to stop moisture/dust getting from the engione compartment into the boot.
I re-used both the positive and neg cables. I removed about 1.5M off the positive cable saving a bit of weight.
The whole project didn't cost a single cent and probably took half a day of my time mucking around with it. Removing the positive cable that was run to the frunk from engine compartment wasn't fun as it was run up behind the plastic underbody covers and cabletied. Getting those damn ties off was fun without a hoist :)
 
The battery just fits though, by a cm or two in every direction.  A smaller Odyssey or Full River would be even easier to install.
 
Hope that helps?

I think a PC925 would work well in an upright position. The battery I put in from memory was about 190Lx130Wx200H.
My original plan was to use a 925 and possibly the black bracket at the bottom of this page. http://www.extremebatteries.com.au/a/Battery-Hold-Downs
And position the battery like the pic below. Should be as simple as making a flat plate then bolting the bracket to it. Even comes with rubber matting and nuts & bolts. Terminals would be easy to access also.