The thought of a convenient and really waterproof rack pack has been ratteling around my gourd for years. For ten years I commuted with a Jandd rack pack. It was durable, well secured to the rack, and mostly waterproof. There are not many rains in North Texas between July Forth and mid-September. However, when it does rain during that time, even ducks leak.
One day while milling about in Wally World's sportings goods, I spied the MTM Sportsmans Dry Box. It was the right size, had an O-ring seal, molded from some kind of plastic that while rigid, will take a pretty good knock without damage, and was only about $17. My wife said she could hear the gears turning in my skull. A quick stop at the local auto supply provided me with some rubber coated hose clamps, fender washers, screws, and nylock nuts. (Nylock nuts? Ask any Harley rider - they know)
Mounting was really not difficult. Placing the hose clamps (these were 1omm for the Tubus rack) around the rack rails so that the flanges were up, I temporairly secured the box in place with tape and used a Sharpie to mark the hole locations on the bottom of the box. Next I drilled holes just a bit undersized to help the seal. Installing the screws with fender washers through the bottom of the box from the inside helps to keep the inside of the box reasonably flat and the undersized holes keeps the screws in place while adjusting the positioning of the clamps. I soon found that the box was pulled down on the rack rails keeping me from using panniers. A couple of 1/8" spacers lifted it just enough to clear the pannier hooks. A bit tight; but more spacers could be added. (The spacers I used were laser cut to fit - a side benefit of having a business with several laser cutters. Any spacer could be used; but I'm spoiled by having the ability to make strange things from acrylic) A piece of scrap foam cut into an X shape provides padding for your swag. A side benefit of the foam is the reduction in noise and fairly good insulation for cold beverages. A small bottle of orange juice in an insulated can wrap stays cool for quite a while.
Another sneaky addition is the safety triangle on the rear of the box. Naturally I did not want to destroy the integrity of the waterproof shell by drilling holes. Some rare earth magnets sewn into the corners of the triangle match up with 3 more magnets duck taped to the inside back of the box. If the triangle gets dirty, just throw it in the wash and air dry. So far nothing has budged the triangle from the box even when transporting the bike in the open back of my pickup.
Here are some photos which I hope will help. I have since added some Nashbar Daytrekker small panniers that have a web between the panniers making them one unit. I just melted holes in the nylon to match the box mounting holes and re-assembled everything. The panniers will not come off; but until I have time to do any longer trips than 1/2 days it will not matter.