I made this lockable cover for the milk crate that sits on the rear rack of my bike so I can leave my tools and other junk inside the box if I go into a store and to give me reassurance that nothing will fall out on my 10 mile commute. Although optional, I added handles to assist getting it on and off and side rails so I can strap something such as a shoe box on top if it won't sit inside.

Step 1: Materials Required

5-ply (mine happened to be grooved) about 13¼" x 13¼" (mine ended up a little smaller on one dimension though I am fairly sure the milk crate is square and the cover goes all the way to the edge)
About 4 ft of ½" or ¾" quarter round trim to hold the cover in position. In fact the scrap I had was ½"x¾". You could use square or even rectangular cross-section timber too but it wouldn't be so neat
About 2ft of 1" x ½" timber for the side rails
A cabinet lock. I thought the one I bought was 1" from Lowes but I don't see it now so this cabinet lock from Home Depot looks good.
16 x ¾" thin (#10?) wood screws, or similar, to secure the quarter-round timber
6 x 1-1/8" drywall screws, or similar, to secure the side rails
2 x cabinet handles from Ikea, Home Depot etc
1 x large nail (I had a length of 1/8" stainless steel rod from something and stainless or brass is better if available)
two-part epoxy to glue the nail into position
paint/varnish
weatherstrip or similar

Step 2: Tools Required

Saw for the timber. I used a hand saw but it would be so much easier if you have access to a table saw or one of the power saws on an arm which might be called a radial saw.
Flat and round files to smooth the edges of the timber after cutting. An orbital sander would be great too. I think I also used a plane since I can't cut straight.
Not shown but I believe I used a Dremel type tool with a sander bit to trim one of the quarter-round pieces around the lock though if desperate the round file will work.
Also not shown, a hammer and chisel, again to trim around the lock.
Power drill and standard drill bits. I've shown my cordless drill here but to cut the big hole I believe I used my wired drill.
Countersink drill bit not shown but I used it
¾" hole saw or spade bit for the lock
Hacksaw to cut the nail
Big and small Phillips head screwdrivers.

Step 3: Cut the Top

Rather than me tell you the dimensions to cut, put the milk crate upside down on the 5-ply and mark out exactly the size you need. Cut the timber, round the corners and smooth the edges.

Step 4: Install Quarter Round to Stop Cover Sliding Sideways

If you measure the inside width of the milk crate, you'll get a rough idea of how long the piece will be but that measurement will be an over-estimate. In my case it was 11½" and you can see in the photo that I've cut them at 45° and rounded off the corners.

This step is trickier than it first appears as you want the quarter round installed so that the vertical side is facing outward, close to the edge to stop movement but not so close you can't get it in and out. I think I taped some cardboard, of the thickness used in a shoe box (1/32" or less than 1mm), inside each side of the top of the milk crate, held the quarter round trim firmly against the cardboard, lifted it out to center it lengthwise, then while still holding firmly, carefully drill a hole through the 5-ply into the quarter round trim so it doesn't move and then insert a screw to secure. I found it necessary to curse often at this point. Once you are sure of the final positions, secure properly - I used 4 screws on each quarter-round.

When I install screws into wood, I drill one hole the width of rod of the screw to full depth, a second hole in the outer piece (in this case the 5-ply) that i wider than the screw so you can push it in, and I counter-sink the top so the screw fits flush. All told it is a lot of changing bits but it does a decent job. If you are more skilled than me, you may just need one hole and power the screw in so it creates its own countersink.

Step 5: Paint

At this stage I removed the 16 screws and gave all the pieces of timber two coats of polyurethane clear coat that we had leftover from doing the floor. You could also use primer and paint, preferably exterior grade.

Step 6: Install Front Pin

This pin is on the opposite side from where the lock will be installed and when you reinstall the milk crate on your bike, the pin will be under the cover under your seat.

Install the cover on top of the milk crate. Get a drill bit the same size as the big nail and drill through the mild crate into the quarter-round trim.

Now use the hacksaw to cut perhaps ¾" from the nail and smooth both ends. I used a larger drill to drill perhaps an 1/8" deep on the outside to create a bigger area for the two-part epoxy to stick too though I'm not sure this was a good idea. In any case, insert the rod into the hole in the quarter-round so it is flush on the inside and glue i position. Since you drilled the hole for the rod through the milk crate, you can be sure the rod is in the correct position.

Step 7: Install the Lock

Remove the quarter-round at the rear where the lock will go.

Use the hole saw or spade bit to cut the hole for the lock. Mine required a ¾" hole and it was set back from the edge by 1-3/16" but please check that will work for you. When the lock is engaged, the arm that sticks out should go through the handle of the milk crate and preferably be flush on the outside.

After drilling the hole, press the alignment plate into the 5-ply. Notice that the straight edges are parallel with the outside. Slip the lock through the 5-ply and the alignment plate and install the nut to secure. Use a wrench to tighten it firmly and the pins in alignment plate will press into the timber to prevent it turning.

The lock comes with a shaped ring thing that allows it to turn just 90° and you will probably want to install it as shown.

Screw in the arm. If all goes well, the key will be aligned with the arm and I have set it up so it is clockwise to lock.

Install a short strip of weather stripping or something compressible. When locked, you need the arm of the lock to be pressed firmly against the underside of the handle of the milk crate or it will bang about and shake itself loose.

Step 8: Shape the Quarter-round Trim Around the Lock

This is a pain but for the lock to be close enough to the edge, it will interfere with the quarter-round trim so I first used the hacksaw to cut about an inch either side of the lock and use the chisel/hammer and flat file to remove perhaps an 1/8" on the face adjacent to the 5-ply so that it clears the alignment plate and securing nut.

Then I used the Dremel with sander attachment to cut space for the body of the lock.

Of course, you could save yourself trouble and just cut out the middle 2"/5cm but it wouldn't look so nice.

Step 9: Install Handles

You will probably want the handles no further back than the front of the lock so that if anything is strapped to the top then the handles will stop it from riding up over the lock. In practice I think the screws are just inside the quarter-round piece so to position the holes for the screws, hold the handles in position and use a pencil to draw a circle around the bases. Then use a nail or hole punch to mark the center and then drill holes big enough for the screws to pass through.

Step 10: Install Side Rails

Cut to length. I made them go not quite to the rounded corners which I think was 11¾" long.

I used a plane to round off the top though that is optional.

Position at the edge and use the 1-1/8" drywall screws or similar to secure from underneath.

Step 11: Mount the Milk Crate Back on the Bike

Make sure the side with the hole for the pin is at the front under your seat so that the lock is at the very back. You are all set then to go shopping, ride to work or possibly ride from DC to NY.