Mystery Ranch NICE frame options

Hey!

I have a Mystery Ranch Mountain Ruck with the NICE frame, but I'd like to add some versatility by taking the ruck off and switching it to a 30-40 liter 2/3 day assault pack when needed (or rather when the 80 liter capacity is not needed). The Overload would be perfect, but MR doesn't have a bag only option and I don't want to pay upwards of a 1000 beanz for the whole thing, when I already have a frame.

I am looking for suggestions on what pack/bag would fit this purpose.