Here is the frame for my trailer after adding 3 more cross-members.  Metal bed rails are strong enough and can be found for free at metal recyclers or the local sanitary landfill.  My trailer had a short 4 foot long tongue.  That I replace with an 8' piece of 2" square tubing, 0.180" thickness. To ease later construction, the trailer hitch is installed and wiring (see below) threaded through the tongue before the bed is attached.

Most small utility trailers are supplied with 8" wheels.  They are too small.  I purchased 12" wheels.  You can see the difference in the photo at right.  Also see the frame with only 3 cross-members and compare it to the photo above as modified with 3 more cross-members.  All construction will take place with the 8" wheels on the axles.  When complete the 12" wheels can be installed.  The 12" wheels can also be balanced.

At right is the frame for the bed of my trailer.  The rectangular section is 6' 6' long - the length of a common air mattress.  The V-nose section is 3' 4" long for a 38 degree V.  I used pressure treated 2 x 4's and 1/2" plywood.  The plywood was screwed and glued to one side. Screws, not nails.  Nails can back out.  Every surface of the wood except the top was painted so glue would adhere properly, than plywood glued and screwed to the surface shown at right.  Painted wood lasts at least 10 times longer than unpainted, even if not seen, so it is worth the trouble.

The bed is set upon the frame, then the tongue weight measured using a bathroom scale.  Slide the bed back or forward to achieve about a 50 pound tongue weight.  When the framing and siding are installed the tongue weight will be about 80 to 100 pounds, ideal for towing. Once positioned, I used carriage bolts to secure the bed to the frame and cross-members. I wanted a flat floor, so first the holes for the carriage bolts were countersunk using a 3/4" Forstner bit wood drill.  After bolting the bed to the frame, I filled the space around the carriage bolt heads with a paste mix of Elmer's Wood Glue and sawdust.  That mix takes two days to dry.  Then it is sanded smooth with a belt sander.  After painting it is impossible to see where the bolts are located!  Presto, a perfectly flat floor.

At right is a photo of the V-nose framing.  I used 4 foot high 2 x 2's for the framing.  A shelf was built to add rigidity to the nose.  Note that the sidewalls of the nose extend back one foot into the straight sides.  When covered with a piece of 1/2" plywood, the sidewalls are tied into the nose section, adding rigidity to the sidewalls. The remaining uncovered top section can be left as is or a cover can be made from 2 x 2's and 1/4" plywood.  Add a back door and the trailer is virtually waterproof and lockable.

The photo at right must be enlarged to see details clearly.  Look at the near side and you will see three studs side-by-side.  The opposite sides have two studs with a spacing between them.  That center stud pivots on a bolt at the bottom and swings parallel to the bed, locking into place in the gap on the other side.  That prevents items inside the trailer from sliding forward or backward when under tow.  There are three of those braces, one at the rear, one at the front, and one marked with the pivot. Also notice all the tie-down eye-bolts.

Here the framing is complete.  The 1/2" plywood top over the V-nose extends back a foot onto the sidewalls.  The front of the V-nose is 2" lower than the back to shed rainwater and lower wind resistance. Note the construction strapping to add additional strength to the sidewalls.

Wiring for trailer tail lights is first threaded through 5/8" clear vinyl hose, then put through the tongue, before the bed is bolted on.  It's easier that way.  Then the wiring can be ignored until the framing is complete.  For my trailer I purchased LED taillights because they use very little electricity.  Even a small car with a 4 cyl. engine will have a large enough generator to use the trailer lights without draining the battery on long trips at night.

The photo at right shows the wiring coming up through the bed and stapled to the stud walls.  Here I used 5/16" clear vinyl tubing because only half the wires go to each side.  With this system the wires from plug-in on the vehicle to the taillights are all encased in vinyl tubing.  Trailers bounce and vibrate, but with wiring encased in tubing there is nothing for the wires to rub against to break or short out.

The photo at right shows all but one piece of exterior skin applied.  All of the wood was painted except where wood that would be glued to other wood.  The sidewalls are cut to accept the fenders.  The screws used to fasten the 1/4" plywood to the studs were countersunk.  Then the holes are caulked and sanded smooth.

Here is the body with 1/4" plywood sidewall skin painted.  Now the fenders can be installed, and the new 12" wheels.  Trim can now be added, such as tie-down hooks.  The taillights can be installed.  The trailer can then be used.  I'm going to add a cover, hinged at the front, and a back door.  Then the trailer can be locked up as well as being virtually wind and water-proof.