I dont know what you are asking, but I'll try infer what you mean, as the box itself is quite small already compared to xyz in the market.
You can put any rda on top, even little 14mm ones. The one that is supplied, gratis, is a 22mm item with a big drip tip etc. But feel free to take advantange of the open source, standard, 510 connector fitted to customise this luxurious item to your personal requirements.
Thanks for the info, but meant, like the Reo has a grand and a 'mini'... looking for a bottom feed device that takes 18490/18500 batteries.