These are my go to buttons brackets:
https://www.thingiverse.com/thing:2942934
But with the buttons you have linked, I have used this one as well:
https://www.thingiverse.com/thing:2865730
But, with those the cartridge reader would no longer fit. I have seen some on the battery cover, check this link out:
https://sudomod.com/forum/viewtopic.php?t=3054
The only problem with using the battery tray, is that you will need to likely mod things a bit more to fit the battery. If you plan to use the cart reader the battery can't go in the cart slot, and if you have buttons in the battery bay, it will be hard to fit a battery in there too.
If it was my build, I would skip the cartridge reader, and just orient the pi with SD access in the contrast slot. Keep the battery tray intact for the battery, and then just use one of the linked button brackets above. (If you are interested in any prints I can print them for you if you like, i'm based in Canada.)