The battery holders are actually very good and work well. For a club racing environment I’d thoroughly recommend them. They are shown below attached so the cells are in the forwards position.
As shown in the manual, the battery trays need to be cut at the corner to give clearance for the T bar.
The team drivers pretty much all have their cells in the forward position so it is only necessary to cut the corner in order to mount the trays that way round.
There are a couple of problems with the battery holders in that they take up a lot of space on the chassis, and they add weight. this is why you will rarely see them on a car at a national. Tape will be used to hold the cells in instead. This is fiddly but overall about 15g lighter than using the holders.
I also use the same tape to hold the battery trays in position, just so they don’t fall out when working on the car. The cells are used in the forward position so often that I’ve know drivers to glue the trays in!