I agree with the idea of just bonding the tube in place with a epoxy, I would still use copper tube for the thin walls. Would suggest you use a but of 200 grit sandpaper to rough up the part of the tube to be bonded into the plexi top.
If you want to use clear as BB said another couple sources you could check.
www.mcmaster.com At this one the platics are in the raw materials & springs section of the first page.
www.delviesplastics.com This ones good only if you intend using acrylic.
Would suggest use of polycarbonate (Lexan) instead of acrylic, it's worth the extra cost as polycarb is about 30X stronger than acrylic.