The sunniest area in our back yard is right up against the garage door..
But we couldn't plant a veggie garden there, so I felt the next best thing would be a movable cart!

I liked the idea of catching and reusing the water it would consume.

I liked the idea of being able to move it around - in the shade on a super hot day or into the last bit of sun in the late afternoon..

I had several ideas for it, including a metal frame.. 6 pneumatic wheels.. a road train with several trailers.. sun shelters.. a rain catching roof.. a green house.. areas to hook tools..

In the end, this was the quickest I could come up with and we were eager to plant as it was getting late in the season..

Step 1: Parts List

Tools:
electric drill and philips driver bit
belt sander
saw bench
jigsaw
sanding block

Materials;
-1 sheet of 2.4m x 1.8m construction ply
-1.2m x 38mm hardwood dowel
-5 x 2.4m x 4cm hardwood garden stakes
-self-drilling wood screws
-2 x plastic wheels
-two sets of billy cart 1/2 inch axles
-exterior grade wood glue
-weatherboard grade silicone gun
-outdoor primer left over from painting the shed
-outdoor paint left over from painting the shed

Update:
-I forgot to mention I used pond paint sealer to seal the inside. Please report back if you find something better!

Step 2: Preparation and Construction

I cleaned up the garden stakes a little with the belt sander. Taking off rough edges.

I used the garden stakes as an internal framework, giving the box some extra strength.
They were cut and used as a brace in all internal corners of the box.

I cut the ply into pieces with the saw bench, giving me parts for a box that was 2m long x 1m wide and 40cm deep.

These were the rough dimensions I kept seeing in gardening books for "a garden bed" and since this was all I was going to have, I figured I should make it as big as possible.

At one end cut the corners out of the main panel and ran the garden stakes right through the panel, creating legs for the handle-end of the cart.
The initial construction pictures don't capture this well. There are more of the legs later.

All joins were glued and screwed.

Step 3: Adding Framework for Wheels

I had some offcuts of plywood, but nothing substantial enough to make a frame the size I wanted, so I cut several pieces the same size and glued them together, adding a few screws to provide extra strength. (see picture 1 and 2)

The wheels were attached by gluing the frame and screwing down into it through the base of the box.
(see picture 3)
Note: picture 3 also shows the legs of the handle end of the cart.

I then began filling in all of the screw holes so that the paint would have a nice smooth surface to run over and not create any areas to attract moisture over time. I couldn't find any decent exterior grade wood fillers, so I ended up using sawdust and exterior grade wood glue.
(see pictures 4 and 5)

Step 4: Adding a Handle

I cut the end off our old mantle piece (it was just asking for it, sitting in the back of the garage there!)

The offcut was about 40cm x 20 cm by 4cm thick.

I cut it roughly diagonally and drilled holes in the thicker end of each piece.

I screwed the pieces of mantle piece to the cart and slide the dowel into them.

Step 5: Painting

I used weatherboard sealant-grade silicone and ran a bead along all internal edges.
(I couldn't find any decent fillers and this was supposedly good for sealing against wood).

I then painted a white primer inside and out and finished with two coats of exterior grade paint.

Update:
I initially forgot to mention that I have used several layers of clear pond paint on the inside.

I'll try to summarise the details:
I had Pond paint left over from a fish-pond-in-a-wine-barrel project that I never used it for (turns out the fish were okay in wine-soaked wood and the wood in the wine barrel expanded enough to hold water!).

The pond paint container lists it as non-toxic and suitable for constant exposure to wet areas.
I wasn't 100% convinced that it was perfect or perfectly safe to grow veggies in, but a> I had it sitting there and b> I didn't find any better alternatives for looking.
Similar commercial applications I found used tar to seal wine barrels. Tar doesn't smell great to me, so I can't imagine it being great for your food to grown in.
Thanks to MikeyNCat for reminding me to post this update.

Step 6: Drain Hole and Tap

I then drilled out a drainage hole and added some small gauge pipe with thread.
(see pictures 1, 2 and 3)

Before filling with dirt, I put some excess weed matting I had from another job inside.
(see picture 4)

Step 7: Adding Dirt and Plants

I used the no-dig gardening method to fill the cart.

I figured it would be lighter and provide plenty of organic matter.

There is a lot of internet information out there on this, so I won't go into it other than to say I used something like:
a layer of lucerne
a layer of fertilizer
a layer of pea straw
a layer of fertilizer
a layer of dirt and compost
a layer of sugar cane mulch

Step 8: Final Thoughts and What I Would Change, What I Would Add...

It worked pretty well, plants grew and we were happy - but there were a couple of issues and things I would change.

It is heavy-
So heavy in fact that the rubber on the first set of wheels I bought (rated at 70KGS each) ended up warping off the wheel. The rich soil holds a lot of moisture - good for the plants but bad for your back.

The drainage hole is good and allowed us to capture and recycle the water, but..
Beware that dirt, compost and fertilizer creates a pretty potent brown liquid that is very smelly and attracts flies. BUT.. you can dilute it and reuse it. Saving that precious water.

I very quickly added a tap so I could control when/how it was drained.

If I did it again, I would make it shorter and possibly narrower. Say 1.5m x 80cm.
This would fix issues with weight.

I would change the design so the wheels were larger and on the outside, rather than underneath - making them more of a feature than an afterthought.

By the way, I always had some other ideas to add to the cart, but have not yet put them into play:

A frame work over the top to assist with netting (to protect from birds) or a tarpaulin (to protect from the sun or heavy rain).

Fold out side tables for potting, picking or your cup of tea.

Tool hooks for holding tools while you are working.