If it was me- ( and I do not have a Bikini pack); I think I would just slip a piece of foam pad that I always have with me for sitting ( either a piece of Z- rest or ridge rest) into that space. It would be plenty wide enough, and could even attach some quick release cord on the foam to attach firmly to stays. I already carry this foam around with me for sitting anyways. Now if my pack contents, or even meat after having that foam placed there was somehow able to still push into that slot and push on to my back enough to cause a problem for me- that would have to be something I would have to see to believe. Because I do not see how it would cause a problem for me with a Thermarest foam pad.
But since I do not have a Bikini- this is still some conjecture on my part.
this is what i did. makes the pack nice to lean against for glassing..and naps.