I used to use the soft side one like this one. The coolers worked ok. but the plastic things that hold the straps tight (that hold it to the bike) broke after being rearended a few times. Now it sitting on a shelh in my garage.
Currently I used the Cajun Dri-Stor. Its a it smaller, but its actually nice to lean back against. Its held on by U-bolts and keeps things dry unless it gets totally submerged.
The soft sided one in black is $70 from ATVDirect.com and the Cajun Dri-stor was $90 from HL (but you can get it cheaper else ware