If you want a good boxmod, I'm sure there will be a ton of suggestions that are all good.
If you want the BEST boxmod, buy a REO Grand. It's not necessary to be able to rebuild, as the REO was designed before RBAs existed. It's a bottomfed box, so there's 6ml of liquid on-board to "squonk" (it's a technical term) liquid up into the atty or carto of your choice. It uses an 18650 battery of your choice. It is a mechanical, so not circuit to fail and leave you stranded without a vape. It's a bit pricey, but it's dern-near indestructible and totally worth every cent. It took me some time to justify the expense, and when I finally took the plunge, I regretted all the time I wasted with other hardware. Many swear by it as their all-day rig. I'm one.
Check out the REO sub-forum for the most helpful, friendly community and support system of REO owners (Reonauts). Come on in. The weather's cloudy, but we like it like that!