So we're out in the backwaters, and our PF is pretty crap, so normal Acquisitions are hard to make. So my Freebooter has picked up a few ranks of Trade (Armourer) and now I'm trying to be "helpful" to my Kaptin and his crew.

Apart from Snazzguns, what could a creative Mekboy make for a human crew? Human-sized Mega Armours?

Whatever your GM lets you make - that's how the crafting system in Rogue Trader works, unfortunately. Assuming he'll let you make anything you can name, I suspect Kustom Force Fields, Warbikes/trakks/buggies and a Tellyporta would be the most useful. Stikkbombs are also quite useful if you're low on grenades.

Depending on how strict your GM is with the Ork tech is unreliable rule I'd have him either make everything or nothing.

Some GMs rule that Orks can make tech that works perfectly if they succeed well enough and if they roll poorly the tech still kinda works but only for an Ork (sometimes only for the Ork that made it).

I'd skip mega armour even if the Ork can make it perfectly. Orks are not well known for empathy or considering the fragility of other races so the first time a non Ork swings a punch in that mega armour they may find their shoulder dislocated by something an Ork wouldn't even notice.

The teleportation pack in Into the Storm (Page 140) seems like something Orks could make and use and they're known to understand force fields pretty as almost second nature so unless you like ork firearms I'd probably stick with that.

That said who doesn't want to be weilding a fully kustomized burna?