For glueing plexi, use Weld-On.
I have Weld-On 16 (i think) but it's a tiny bit thicker. I think the Weld-On 40(?) is the thin stuff. You can apply this on the edge of the joint, and it will fill in due to capilary action & glue the pieces together.
IPS Weld-On 16
http://tapplastics.com/plastics/adhesives/444_wo16.html