no1mad 

Okay, I've got this bike that has 16.3" chainstays. I've had heel strike with all of the lower cost panniers that I've so far tried. Now have a trunk bag w/fold down mini-panniers. It fits my rack beautifully, as in there is no overhang off the back. However, it's a bit too small for my job hunting outings. I use of those document storage clipboard things to transport applications and resumes without them getting all wrinkly. None of the compartments are large enough for the clipboard.

So now thinking about another, larger trunk bag. Only thing is, I don't know how much 'wiggle room' there is in regards to overhang. Back when I ran with my beverage crate, I wasn't as concerned, because I knew that plastic wasn't gonna flop around back there. Rack trunks may have some rigidity with their foam panels, but if one were to overload, I'm not to sure.

lasauge 

I'd go for the larger bag, if there's too much flex you always can add a DIY stiffener made of coroplast (the stuff they make political ad signs from).

hubcap

I don't know what trunk bag you have currently, but some of them come with expandable collars so they can accommodate larger loads. The problem is most I have seen unzip and expand along three sides (the back and the two sides) while leaving the top front of the bag, nearest the saddle, fixed as the pivot point. That would probably still be an issue with your 8.5x11 shaped load. How about just strapping the 8.5x11 thing to the top of the bag? Is it just a clipboard, or is it a box that papers go in? 2-gallon ziplock around it to keep it dry?

no1mad 

I have an Axiom trunk that expands up as well as down with those drop down panniers. The panniers aren't wide enough. When expanded up, I can place my clip board thingy* in at an angle, but I can't zip the bag shut.

Linky to thingy. I keep a supply of resumes as well as SS card and birth certificate in a ziplock baggie, a couple of pens, and whatever applications that I've picked up or dropping off inside.

hubcap

That is what I was envisioning. I would consider putting the whole thing in a big ziplock back and strapping it to the top of the trunk bag. Or you may need to consider a backpack/messenger bag.

Also, I have had luck with using a long rack and panniers with adjustable clips to avoid heel strike on a bike with 405mm chain stays. Switching out racks and panniers is likely a lot more costly option though.

no1mad 

Yeah, the least cost option at this point will be for me to go back to using the backpack(s). Now I just gotta make up my mind on whether I wear the pack or strap the bag to the rack and reconfigure my rear lights accordingly...

Chris_in_Miami

I have a gigantic Axiom trunk bag that I mainly user on my Moulton's wide rack, but it sometimes goes on my other bikes. I've noticed that on the standard racks, it hangs down over the back if I load the rear pocket with my tire kit & multitool. It would be a problem if I mounted my tail light on the rear of the rack. This bag also hangs over the sides of the rack and looks pretty sloppy, but it's insulated,has a waterproof liner, and holds a twelve pack!

If I were to use this bag exclusively on the standard width racks, I'd definitely want to stiffen the bottom with a coroplast platform.

fietsbob

+1, Lay in a piece of coroplast on the bottom, for stiffness, if there is some overhang.