Some of the Lowepro Flipside Backpacks bags open on the strap side to avoid just that problem....but it limits the open size a small amount.Would prefer something that didn't have to rest on the straps in the mud but oh well:
http://www.bhphotovideo.com/c/produ...P35271_PEU_Flipside_400AW_Backpack_Black.html
Bob