Very true. However, f-class is literally Benchrest from the prone. Thats what the rules dictate. 99% the same game IMO. I have shot service rifle. F-class shares almost nothing with high power.
Not trying to piss in everyone Cheerios here, but as recent as the NRA HPR rule book of 2014, under 22.F-Class, 3.4.1(a)(2) paragraph three, "This discipline is a modification of high power prone shooting, not a form of bench rest and should not be construed as such."
Just sayin'. With the recent bungling of the service rifle rules updates and general lack of crosschecking referenced rules it is clear the NRA needs to sub-contract the compilation, clarifications and editing of the rule books out to people that do this for a living.
Off my soap box and back to the OP. Why not just use one of the doughnut shaped bags, partially filled, under the rear rest?
Cheers,
Jim