I've heard good things about Spencer ...hard to beat a custom seat.
I've had good results with the Airhawk products. Had one on my bike during the 2300 mile trip from NYC to Tennessee (and back) for ROTR 11. It doesn't win any style points(!) but it is effective. It comes with straps to attach on top of your seat but I dispensed with the straps and simply attached the Airhawk using a couple of velcro strips.
Amazon.com: Airhawk Medium Cruiser Pad - 14in x 14in , Size: Md AH2MED: Automotive
I'm assuming you are travelling on highways and interstates on the longer trips?
I find the zr-7s to be a VERY GOOD highway bike. There's a zero gravity double bubble screen on mine. Hard to imagine that a small screen like that can actually make a big difference. The air simply flows over my helmet at around 80-90 mph on the interstate. The bike also has virtually no vibration at that speed. Also, with the extra weight (525 lb) that the zr has, tractor trailer dirty air doesn't really bother it as much as a lighter bike (I make it a point not to stay behind the tractor trailers for long...) Oh, also, if you take breaks every few hundred miles, it should help with blood circulation.
As with anything, riding longer distances becomes easier with practice.