new to the forum... grad student, loyal dc urban commuter (saddened as well to hear about the recent death on 14th and constitution)... i'm on campus all day and need to figure out a creative way to lug books, lunch, snacks, work-out clothes, etc... and sometimes even dinner and breakfast... have a slim, narrow profile backpack that i think i can modify to attach to an old rear rack i have... do any of you do-it-yourselfers have any tips for whipping up some sort of homemade clip/attachment system...? would love to get a nice carradice or rivendell saddlebag--even an arkel bug--but it won't be in the budget for another two years! thanks...

Yep. Get hold of a set of Arkel clips... they're sold at $20/pair so you can retrofit your inferior competitor's panniers I did this with a picnic backpack and was very satisfied with the result.

Alternately, Nashbar has a couple of convertible backpack / panniers on sale right now... the Delta one is just $40.

Another attachment question. I'm trying to hang a waterproof binocular bag on my seatpost. I've added a heavy velcro to wrap around the seat post. How do I get the straps to suspend around the seat-rails without sagging??

I used to use some military surplus respirator cases, zip-tied to my rack. It is a non-removable system.
The Carradice system is very amenable to home-made design. You need a suitable aluminium extruded rail, drilled and riveted through the material to a corrugated plastic stiffener. The Rixen and Kaul clips slide onto the rail. The R&K anti-flap hook is also rivetted in the same manner.
I'm taking my 8year old bags to the local market for a leather worker to re-rivet.

Another backpack -> "pannier" option is a folding basket attached to the rack. Just drop the backpack inside. Bungee it in if you've got rough roads.

this is probably way more info than you need, but ken kiefer's site may have some helpful info.
here a link to his homemade panniers
https://www.kenkifer.com/bikepages/touring/bags.htm
lots of other great stuff on his site too.

I'll check 'em out. The stiffener worked well and gave shape to the bag. My bag weighs 2 oz (compared to the JANDD @ 2.5 pounds. It is bigger and holds the flat kit and bike chain.