This Ible describes a grass/leaf catcher that sits on a platform attached to the rear of the mower. This is the same idea, different approach.
Step 1: Can Your Mower Use a Grass Catcher?
If your deck doesn't have an opening then you are going to have to cut an opening to make this work. That is a much bigger project and will leaved your mower permanently modified.
Step 2: Remove Any Covers
Step 3: Fit Blind-End Tee
Position the tee as shown (Photo #1) and begin to trim away the part of the tee that block free flow out of the deck. Use a sharp utility knife and cut away a little bit at a time until the fit is tight (Photo #2). The tee should overlap as much of the opening in the deck as possible. Leave tabs as you trim the tee so bolts can be used to hold the bottom in place. The tabs should be positioned on the outside of the deck to maintain clearance for the blade on the inside.
Use bolts through the top of the tee to hold the tee on (Photo #3). All bolts are 1/4" carriage bolts with the heads toward the inside of the deck. The nuts and lockwashers go on the outside to maintain clearance for the blade on the inside (Photos #4 and #5).
Cut a piece of sponge to fill the open top part of the front tee (Photo #6).
Step 4: Make Rear Platform Frame
Make the two outer bends at the 16" marks and check that they are parallel (Photo #3). Then make two bends at 90 degrees at the 33" marks (Photo #4 shows first of these bends). Photo #5 shows the completed frame.
Step 5: Attach Rear Platform
I tried two different ways of making the lower eyebolts for the frame (lower two eyebolts in Photo #4). One way was to use a second smaller eyebolt secured to the larger eyebolt with two locknuts. The other way was to bend a 6" long 1/4" eyebolt 1.5" from the end (Photo #5). Either way these were attached to the mower frame and then inserted into matching holes in the platform frame (Photo #6).
The platform I made was 17" front-to-back and 20" wide. Almost any piece of waterproof plywood will do. This was bolted to the frame with four 2" long 1/4" hex bolts and locknuts (Photo #6).
Step 6: Mount the Outer Garbage Can
Use a utility knife to cut off the top flange of the outer can (Photo #2).
Screw the outer can to the platform with coarse thread exterior (drywall-style) screws. Use small pieces of plywood to support the bottom of the can and reinforce the attachment area (Photo #3).
Bore a ~1.5" hole all the way through the reinforcements and can bottom so air can flow in and out easily (Photo #4). This will make it easier to remove the second can later. Remove the handle from the outer can.
Step 7: Add Second Tee Attachment
Use a ~2" hole saw to bore a "thumb hole" about 2" below the big hole you just made (Photo #2). The smaller hole will let you push in on the inner can to make it easier to remove.
Cut a length of the drainpipe that is about 6" long. Gently force the pipe into the round hole in the side of the can so that the pipe snaps in place in the first groove (Photo #3). You might need to cut short slits into the first ring of the pipe so it can be slipped through the hole more easily. The one ring on the inside of the garbage can should hold the pipe in place.
Step 8: Adapt the Inner Garbage Can
Use a felt tipped pen to mark the around inside of the outer can pipe stub. This will locate where to cut a circular opening in the second (inside) garbage can. Remove the inside can and mark a 7 1/8" circle centered on the circle you just drew. Cut this hole out of the side of the inner can (Photo #1).
Now when you slide the inner can down into the outer can the 7 1/8" hole will snap over the inner ring of the pipe stub. This will hold the inner can in place. To remove the inner can press through the thumb hole and slip the inner can over the pipe stub.
Step 9: Connect Tees With 6" Pipe
Measure a length of drainpipe long enough to extend from inside the lower forward tee to the inside of the rear tee. Fit the pipe into the tees at each end (Photo #2).
Use three cable ties to secure the pipe from the outer can into the tee. Drill two 1/4" holes about 2" apart for each cable tie to loop through (Photo #3).
Step 10: Modify Garbage Can Lid
Next I made a retaining ring out of 14/2 house wire. I cut a length of the wire that would exactly circle the inside of the lid where I wanted the screen to be. I slipped the jacket off one end of the wire by clamping the wire inside in a vise and pulling. When I had about 6" of wire out one end and 6" of empty covering on the other end I stopped pulling (Photo #3). Then I slipped the exposed wire into the empty jacket to make a continuous circle of wire. I taped the joint and the area over the inside joint of wire with electrical tape (Photo #4).
I cut a piece of plastic window screen about 4" bigger than the garbage can lid in each direction. I put the screen in the lid from the inside and secured it in place with the wire retaining ring (Photo #5).
Step 11: Test and Enjoy
Step 12: Modify Rear Tee for Cleaning (Optional)
Step 13: Parts and Tools
2 ea Blind-End Tees for 6" Drainage Pipe
10' of 6" Drain Pipe (solid, not perforated)
10' of 3/4" EMT Conduit
4 ea 6" X 1/4" Eyebolts
Bag of 1/4" X 20 Stop Nuts (need 12)
Waterproof 3/8" Plywood
5' of 14/2 House Wiring
Cardboard for Patterns
Wrenches and Socket Set
Pipe Cutter or Hack Saw
3/4" Pipe Bender
Felt Tip Marker
Sharp Utility Knife
Hot Glue Gun