Sry, I was busy...
On the cuts:
No, I buy them the length of the batt I have (least the longest in the fleet for that truck.)
If you browse their site you can see the lengths. Basically 3,4,5,6,7,8". I buy one that is long enough, and then cut it down lengthwise. They are very wide, so several inches may need to be removed from the centerline.
Hard to explain, but I cut all the way down the sides, then stop at the bottom plate. Remove the amt of material from the sides equal to how much you need to narrow the box.
The bottom plate is left, and cut once to leave "ears" on the now separated pieces. One of these should be roughly equal to the desired width for the tray (ie 50mm or whatever.) the other side can be just shorter, but not too short, as you will overlap these pieces and apply cement to lock the halves together in a strong bond.
When I make these for truggies, I make them so the bottom does not so all the way across, so it leaves a space for the mud guards. (illustrated in diagram.) This would have to be roughly measured for the trug its going in for how much width is needed. The thicker bottom plate is helpful too to countersink the screws and have a strong base to attach screws. If need be, a small scrap is cut for the end walls and is cemented in. You don't want to just cement edges together or it will never hold. Foam blocks can be added for batts as needed (seen in the box for the .5)
Don't get too wrapped up in exact measurements, it just needs to be close, and test fit the pieces on your batt before cementing. trim where needed first. If an edge is not totally straight, doesn't matter as that's the overlapping is for. Once its all together and cemented, I then use a hacksaw (good for plastic) to cut the box down to the desired height. Lastly notch out a slit for power wires if needed.
If you need to, I would cut them in half width wise to make two "cups" but a single piece is better. The one above is a 7" box to fit a 165mm Neu lipo. Its tight.
You want to use ABS, nothing less. ABS cement will melt and rebond the plastic together as a single piece. Glue is just sticky and will come apart in a hard crash. A hacksaw, a knife, ABS cement and perhaps a drill (easy to remove standoffs w/ drill) is all that's needed. A sharp wood chisel is also helpful to trim the plastic in places if you have one.
Empty box:
