I have got a good one from Freebird Conversions. They supply it as a Gas Bottle Locker which comes in black and can be locked with a padlock. I placed it underneath my rock'n'roll bed and put the bolts coming up through the ply floor into the box so it cannot be removed unless you get into the box.
As it is big enough for gas bottles I can not only get my laptop in there but loads of other stuff as well.
I think the box is around £99 but say you spoke to Matt off of the forum any you may get a bit extra discount.
hi there i have a swivel seat base in my t4 with a built in safe from CMC it works a treat, think they would do one for your t5, i put all my valubles in it small laptop flat screen tv and playstation, sat nav and as my insurance does not cover eletrical items its money well spent,T: