I have since made another sluice which is much better. See the following link: http://www.thingiverse.com/thing:1784607
I quickly knocked up this small (but can be made any length just by printing off more than one middle riffle section) simple drop riffle style stream sluice which is to be 3D printed in a minimum of 5 sections (2 sections make up the flare and a minimum of 3 sections make up the sluice run). The sections have been designed to be glued together with ABS slurry. so hopefully once assembled it will be quite sturdy.
With no mats or anything like that to worry about, cleanups will take only seconds. Spend more time sluicing and less time cleaning up and find more gold!
It is small and has a relatively wide flare so it is good for low flow situations, or great for the kids, or anyone just starting out in this fine hobby.
The assembled sluice is can be whatever length you like, with a 25cm wide flare. The straight sluice section is about 14cm wide. The walls are 4cm high.
Video of the sluice in action: https://youtu.be/TbBDW6vayE8