The Burandt Tunnel bag sits directly on top the tunnel w/4 lock and ride clips into the grooves. I have used the Burandt bags (Worn one out and bought another) for maybe 3 or 4 yrs. Love 'em. Easy to configure and organize, sits tight to the tunnel (no flopping around), low profile. Interchangeable between my Axys RMK (155x2.6) in the Mtns and a SBA ((144 x 1 3/8) on MN trails.
I have not seen a spacer of any kind. Not sure if that would work unless the spacer is a mostly solid plate. The base of the bag is fairly stiff, but not enough to hold the bottom of the bag flat without support under the entire surface. Then if a solid plate, i think I've defeated the intent/purpose.
Overall, haven't noticed any issue with cooling the sled that can be contributed to the presence - compared to lack there of - the Burandt tunnel bags, even on the trails going up to, and down from the good play areas - compared to the scratchers being up/down (which make a huge difference) - far more than having the bag tight to the tunnel could cause.
Great line of thinking, but I feel a non-issue in this case - IMO
