Any tips for cutting or sizing or measuring? I’ll use blue loctight and probably some rubber or foam anti-vibration tape where needed.
Thanks in advance.
Your experience and design will be different to mine if you're using a 500Wh battery rather my longer 625Wh. However, I would advise that you knock up a quick prototype first which will allow you to see where the insertion and removal issues arise, and to fine tune the precise positions for the mounting bolt nuts (or threaded holes) that the two frame bolts will screw into. A couple of pointers:
1 - with the original lock system the battery simply popped out laterally from the frame. This won't be the case with the new bolted system because the bracket that you will make will need to intrude further up inside the frame towards the headtube (because this is where the two fixing bolt holes are). What this means when you come to insert or remove the battery is that you first have to insert the new bracket into the frame hole at the headtube end, then slide the battery up towards the headtube enough to give clearance between the bottom of the battery and the electric connector block that sits at the bottom of the hole in the frame. Now you can insert the bottom of the battery into the frame hole and pull the battery downwards so as to slot the female battery connector into the male connector block in the frame, then push the battery fully flush into the frame. If you've correctly positioned your fixing nuts (or threaded holes) on the end of your bracket these should now align nicely with the two frame holes under the headtube and you can just screw the fixing bolts in. BTW the original bolts here that hold the lock mechanism in place are the Torx type that have a tiny protrusion in the middle of the star hole (can't remember their nomenclature) so you'll need an appropriate Torx key with a central hole in order to get these out. I replaced them with normal bolts!
To remove the battery is the same fiddle in reverse! Once the two fixing bolts under the headtube are removed you'll need to prise the top of the battery cover laterally out from the frame hole a few mm with your fingernails (!). This means that you can now slide the battery up inside the frame in order to pull the male/female battery connectors apart. Now you can pull the bottom of the battery laterally out from the frame hole, and then pull it down so as to slide the bracket at the top of the battery out. Yes, I know this all sounds daunting, and it can sometimes be a bit of a fiddle, but if I've explained myself clearly you will find that once you've discovered the knack it really isn't so hard.
If you look at the photo of my prototype you will notice the odd shaped cut-out in the plate with the retaining nuts. This is necessary in order to provide the clearance around the frame hole to do all this jiggling at the head tube end (because your bracket will be inside the frame at this point while the battery cover will be outside). You can see my red markings to show where I needed to modify the original cut out to give a better jiggling experience"
2 - you mention using locktite, presumably on the two fixing bolts into your bracket. If you don't intend to remove your battery now and again, then fine. But what I did was to cut a small strip of inner tube and wrap and stick it around the end plate of my bracket, over the two retaining nuts. This serves two purposes: 1 - it allows me to fine tune the positioning of the battery in the frame hole and hence the frame gap around the battery cover (i'm a bit fussy on aesthetics); 2 - it means that my two fixing bolts have to screw through this rubber to reach their retaining nuts. This acts as a perfect "thread locker" and prevents the bolts from coming loose, but allows easy removal and re-insertion whenever needed. In over several 1000km with this system my fixing bolts have never come loose and I've not had the slightest rattle or noise from it.
I hope this long explanation is coherent and helpful. It took me a while to figure out the details of how to make it all work properly, but once I'd created the prototype it didn't take long to get to the finished version. Sadly I can't show you a video of inserting or removing my battery because my video format is not accepted here!
Good luck with your project!
). No adapter necessary since the 500wh battery is exactly the same length as the 625wh battery. 
