My husband  Jim has built me a couple of these whelping
boxes and I have had numerous people ask me how we do it.  I
thought I would share the instructions for those who are
interested.
This box is made from 3/8 inch plywood,  and 1"x1" strapping
or nailer strips, it is fairly lightweight,  but strong.  The size
is 30" x30" x 12'' tall.
You will need to purchase a 4'x8' piece of 3/8 inch plywood, 
good one side from your local lumber store.  Lowe's, Home
Depot or Rona are usually very good about cutting it to your
specific dimension for a minimal charge.
This drawing is the one that I take with me and they
cut it while I wait.  Save and print to take with you.
You will also need about 15 feet of 1x1 strapping or
nailer strips, a 24" piano hinge, some 3/4" wood screws, 
a bottle of wood glue and 2 hooks & eyes.
Jim starts with the bottom,
and puts the  nailer strips as
illustrated (right),  securing it
with wood screws and wood
glue.  We turn it bottom side up to reinforce it with the wood
screws. The nailer strips also go up the 4 corners for strength. 
Glued on the inside and screwed  from the outside (see below)
 . 
For the front we  use a piano hinge,  so that mom can easily step over, and she can see where her babies are. 
As the puppies grow, and learn to climb out of the
box,  we put the front up. It can be secured with
eye hooks on either side.  We paint it well in
advance of use (at least 30 days)  with a latex paint.
The front of the box can be put up, so puppies can't
climb out,  but mom is still able to hop in and out as
she chooses
.
Click on any photos to enlarge
.
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