I built one 4 years ago and am building a second to replace it now. I made a few mistakes on the first one, mainly I overbuilt it. 6000# leaf sprung drop down axles dual brakes 16' + 4' dovetail, diamondplate deck and ramps. The trailer weighed in at 2800#. The dove tail would drag everywhere and the darn thing pulled like a tank. I hauled a 14000# forklift and the springs were still 2" from bottoming out.
What I'm building now is 16' flat bed no dovetail, open center, wood deck 5000# drop down torsion bar axles, dual brakes, 8' 4" wide deck with a 42" wide open center probably covered with expanded metal.
Some considerations.
**The wood deck is nice because it's much quieter then the diamond plate. Try dragging a chain across a diamond plate deck at 10:00pm in a campground.
The wood is stiffer than the diamond plate. Take a good look at a used diamond plate deck trailer they are usually bowed in in several spots where something heavy was loaded in an unsupported spot.
**Completely sealed wiring with weatherpack connectors going to sealed LED lights that are fully protected top, bottom, sides and rear. How many times have you lent your trailer out and it comes back with 1 or no tail lights? I completely replaced the wiring in the first trailer after 4 years. I used that crappy 4 wire spool stuff without any protection the first time. This time I ran everything in used air line inside the trailer frame. It took a long time to run but it should last forever. I also left an extra foot at the tail lights and at the tongue for future "modifications".
**Adjustable height tongue. I'm using a bulldog 2 5/16" ball 3 position tongue. It can adjust about 6". Makes it much easier to match the trailer to different vehicles.
**Removable fenders strong enough to stand on in the center with grip material on it. I used the skate board stuff.
**Trailer tires instead of passenger car tires rated for the trailer capacity
**Provision for winch mount on trailer tongue for those times when your buddies rig is inop.
**Permanently mounted spare. I'm using the crank up type mount from an '88-98 Chevy truck.
**Built in tie down rings at 8 locations on the deck and stake pockets at 12 locations (4 on a side, 2 front, 2 rear) inside the outside frame rail.
**Tool/storage container. I'm still working on this. I'm probably going with a tongue box, but I'd like to stick a couple 4-6" PVC tubes underneath for spare parts. I'm debating about a couple of 8" underdeck storage boxes instead of the PVC tubes. I'll have to see what kind of room is left over when I'm done.
My goal is a finished weight at ~1700# without tools or spare parts on it. I've got the frame done with the axles mounted, I still need to add material to mount the wood decking to, stake pockets, winch mount, wiring and fenders. So far I have approximately 35 hrs in the fabrication and 12-14 hours in parts/material acquisition. I anticipate another 30-40 hrs of fab/assembly left and 20 hrs for paint, stain and final assembly. Total cost using new parts is approximately $1300. A similar trailer with crappy wiring, no spare, no tie downs and fixed fenders sells for $1750. Not a big savings for the amount of work.