We've been slightly annoyed at how the trays are prone to slide around too easily. On the plus side, it's easy to move them along the templates (assuming nothing is in the way) but when you move into contact with other units trays can get bumped slightly, or someone hits the table when getting up and so on. Depending on your playing surface things might not be so bad but for us we wanted some sort of solution.
After a little looking around I picked up these at The Home Depot: http://www.homedepot.com/p/Everbilt-1-2-in-Self-Adhesive-Vinyl-Surface-Bumpers-16-per-Pack-49967/203661150
Cut them into quarters and they fit perfectly into the tray corners. They just barely lift the tray (not even 1mm) so it's the rubber bumper that makes contact with your playing surface, not the tray. For a single base unit like a hero I could see putting one on all four corners, but for larger multi-tray units I think you're fine with 2 per tray as you'll have lots of bumpers gripping the table. If nothing else it's up to you... more bumpers = more grip.
$2.97 for 16 bumpers (or 64 corner pieces) isn't a bad deal, although I'm sure similar products are out there too.
