I thought I'd discovered the little secret with these tubes but it looks like you've all already sussed it out!!!
I found these tubes for sale at
Reverse Garbage in Brisbane. $3 for each 1.5m length. I thought they were used for frabic. It makes total sense they were used for window tint now, because there's still a bit of tint attached to one! They also have heaps of those end caps too. And cheap perspex off cuts, benchtops, although most stuff is marketed for the art/craft factor.
As for capping them off, i'm planning on gluing a 100mm glass dome light fitting to the top. Bunnings have these on clearance for $2 at my local stores atm.