Pete,
I went through the same loop myself a few years back.

The ply sailbox was on the trailer when I bought it over 20yr ago and was starting to rot. It had only been painted with enamel and I'd done nothing to it over the 20yrs.
So it was not a bad innings.
One thing that ticked me off with the old box was the hassel getting the ply lid on and off by myself so I decided to design a ply box that would be easier to use and coated it in a light layer of glass and epoxy. I'm execting this one to outlast my Gal trailer.

Below is a description of how I tackled the problem.
I'll try to attach a couple of free hand scretches
to illustrate.

The Sailbox was made from 2 sheets of 1/2inch ply.
If you don’t mind I’ll talk metric. I usually think in Inches and feet but
work in metric because it is easier.
Comes from changing systems half way through my schooling.

Where I buy my ply they also cut it. So why not use the service to save time.
2sheets of 12mm ply 1220mm x 2400mm (1/2 inch ply 4ft x 8ft) make the lid,
the base and both sides.

You need to get a little extra for the front and back of the box.

I had the lid cut 50mm wider that the base so I could attach the sides to the lid which
were just 18mm (3/4 inch) x 45mm (1 3/4 inch) strips of maple.

I think the lid was 850mm wide and the base was 800mm wide. the sides 350mm.
Glued and screwed 18mm x 18mm maple to the edges of the sides as well as 18mmx18mm
for the runners down the sides.
Then glued and screwed the base,front and rear to the maple on the sides.
Then cut the lid a little shorter than the base to accomodate the front watertight locking and
glued and screwed the 18mm x 45mm maple to all 4 sides of the lid.

Glued and screwed a peice of 18x18 to the inside front level with the bottom edge of the 18x45
already attached. Just before attaching this 18x18 I cut a bevel on it so it was more like only
13mm on one side. So there is a bit of lead in to the locking.

Then I modified the front of the box to make the locking mechanism shown in the drawing.
Then I fixed the wheels. The wheels are really neat with 45 or 50mm dia and rubber tires.
They cost me $1.60 each.

When you open the box. You lift the lid. At this point it runs on the wheels at the front of the box.
You run the lid forward an inch or so and then put it down. The runners don’t go all the way to the
back of the box so you are only moving the lid until the rear wheels can make contact with their
runners. Then it just run back and forward with just a light touch.

Also with the wheels on I had to cut a small slot in the side of the box at the rear for the wheels axle
to drop into when the box closes. Otherwise the lid when far enough back to close would be sitting
on the axle which would be sitting on the sides.

This is much harder to explain than do.
Let me know if it is unclear.

Regards,
Phill