I lined the "lid" of my jerry can holder with peel'n'seal from Lowes. My water jug moves some, but the gas can doesn't move at all.
As for it rattling on bumps... When I assembled mine I put some pretty thick 3m double sided tape between every connection. Where the swingout "sits" (by the latch) I put some of this...
Shop Waxman 2-Pack 4-in Rubber Pads at Lowes.com
I don't know if that's the exact product, but any hardware store should sell it. It's to keep couches/chairs from sliding on the floor.
What bugs me are the "sticks" on the stick bolts. I can NOT get those things to break off, so they rattle in the frame rails.
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2006 Lexus GX 470 Sport- RR Diff Lock, Metal Tech Sliders, RCI Aluminum Skids, Front Runner Slimline II Rack, SCS F5s, 255/80R17 ST Maxx , OME BP-51s, Dirt King UCAs, Metal Tech RR Lower Links
2016 Lexus IS300 F Sport- F-Sport Air Filter
RR Racing tuner