Many years ago mu wife - bless her heart - purchased online a set of draws + tool box on casters combo.
The item arrived damaged so wifey complained and a second item was sent pronto, and we were allowed to keep the first one. WIN!
These toolbox combos are NOT a bunnings stock item. Nor are they made of quality materials. Hence after I filled them with my tools then attempted to wheel them into position, most of the caster mounting points bent / caved in.
So after some time I had a spare day to get this little project done and breath new life into my mobile toolboxes.

Materials
140 x 40mm x 1200 timber.
*marine ply would have been fine
2 x 4pks of casters
8 x 80mm self tapping wood screws with counter-sunk heads
32 x 30mm self tapping wood screws with counter sunk heads
Tools
Ryobi cordless grinder 18v + thin 110mm grinding disc
Ryobi cordless driver with Philips-head driver
small drill bit for pilot holes
Ryobi cordless circular saw
Spirit level
Cordless vac for clean up
HPE
plastic eye glasses
gloves
Steps
Step 1
1. Empty the toolboxes and turn them upside down
Step 2
2. Using the grinder remove the corner caster mounting points
Step 3
3. Using the grinder remove any excess metal in the corner
Step 4
4. Using the circular saw cut the timber to lengths as shown
330mm long and place into position.
Step 5
5. Using the Ryobi cordless drill, drill polit holes 2 in each but end of the timber, 16 in total
Step 6
6. Change bits to the philips-head driver and screw in the 80mm self tappers.16 screws in total
Step 7
7. Dummy-fit the casters on each timber corner and drill 4 pilot holes.
Step 8
8. Using the cordless driver screw in the 30mm sefl tappers.
Step 9
9. Repeat steps 7 & 8 seven more times.
Step 10
10. Turn the tool boxes right side up and check for level using the spirit level.