Unless you know a machinist willing to fabricate one from scratch, you'll need to source a drop block made to fit a tube with the same id as your 1358's airtube.
That's your starting point. And you'll need to decide if you to go with paintball threads or metric threads on the drop block, usually determined by the size/capacity bottle you want to go with.
Oh yeah, you'll also need to determine if your tube needs to be shortened, but only after considering where the end of the bottle will be in relation to the end of your barrel/shroud.
A few things to consider for sure.
Hope this helped somewhat?
EDIT: I guess I should've googled Beeman 1358 underlever BEFORE dolling out advice - MY BAD