I saw something like this on the web over 10 years ago for a 914 GT project.  I decided to do this my way and document this.  The idea is to make the tray removable for inspection, cleaning and paint as needed to preserve the car.  I doubt it will sit outside, collect leaves, be left to rot, etc., but I have the model here.  You need to do a bit of welding and obtain Panel Rivets that are M10 sized for this mod.  The Caplan Battery Tray Mod follows...

 

Here you see a stock tray support where the top of the picture faces into the engine bay and the left (clamped) side here is the upper side of the tray.  It is very thin here and I needed more meat to place the weld-in bolt threads.  My donor metal is the top part of the jack plate because I used only the bottom half.  ANY scrap piece of metal that is long enough and about 1" wide would do.

 

 

 

 

 

A view from the back-side.  Again, this is the side of the tray support that is "UP" on the long towards the rear trunk:

 

 

Welded, cut, rounded, and ground-down, it looked like this (more full images of the support below).  This was welded on both sides and ground down on both sides:

 

 

The bolts are M8 X 25mm long.  Heads removed (Clamped in Vise-Grips with a terry towel or double nutted to protect the threads and then beheaded.  Notice that about 1 thread is out, that's the thickness of the battery tray.

 

 

Into the tray for welding.  In later pics you'll see that I cleaned off the paint, but here I cleaned it for welding.  The modified M8 bolts sat on the ground and the nuts basically make them straight as the tray sits on the nuts.  To prevent the nuts from getting welded in from hear, a quick tack ZAP of the gun secured the studs in place and then the nuts were backed-off away from the tray flush to the ends.  Final welding was fast:

 

Welded up - bottom view:

 

 

Welded-up - Top view before girding:

 

 

Here is the bottom mount with the wider tab welded (right side of image showing under the tray mount.  The round ears of the tabs were cut with shears and then lightly sanded with my 90-Degree grinder and a 60 grit pad and then some Emery cloth.  I like to remove the "darts" from sharp metal corners when I work:

 

 

Bolted in place.  Will be using NyLock nuts on the final install.

 

 

Another view with a spare harness mount on it to test my fitment:

 

 

Another view after the wire brush cleaned it up:

 

 

Another view from the top with the welds ground-down:

 

 

Another view (I did weld that tab on from both sides):

 

 

One final view flipped 180 degrees: