I built mine from ground up. When I was in the research & planning stages (before the build) I done lots of measurements of the gear I wanted to store in it all times in my trailer. I spent many hour on line looking at trailers built by the owners on a few forums, trying to get a grasp on style, size & functionality.
Our king size bed made for a good lay out for the size of trailer (tub or explore box) you want by laying butcher string in the size you are thinking of. Then we placed the totes inside the string area & using a tape measure figured how tall \ height the tub or explore box is needed by stacking gear on the tub. Once we figured what we wanted in size, style then the drawings came to play & the research of parts & budget to spend on the build of the trailer were penciled. I found that Scott Chaney w\ Compact Camping Concepts was my main go to for many parts & questions & answers since Scott built more adventure trailers than any other single person I know. His knowledge & expertise is a true blessing for the DIY trailer builder from wheel to tub & explore box spacing, to materials used in explore boxes to books he has written on explore box trailer builds, tongue lengths, weight as a few importance. I went with Scott's Dinoot tub.
I'm currently on another build & buying more parts from Scott because of his thoroughness.
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If I can give one piece of advise, do a lot of research on what style trailer you want, don't cut corners on this build. Set a realistic budget, use the right parts & install them correctly whether by welding or proper fasteners & it will last you & give you great trouble free service on the trail.
Last year the wife & I were in our home state of Arkansas in the Ozarks Sylamore District going down a barely 1 1\2 vehicle wide road & 2 1\2 miles in when we came across a slap together junkered overland trailer with failed suspension left behind blocking the trail. In all reality I wanted to take "Ol Heavy" my truck & push this heap over the edge, but I didn't & I had to back up w\trailer up hill for 2 1\2 miles on a windy dirt road which took forever.
My actual build of my 1st trailer can be viewed on the American Adventurist forum.
my project I went all out & over my budget, but I know I will not have any issues selling this trailer this winter once I finish the squareback teardrop style.
Size of my M416 Dinoot is over-all length 11ft. 9", 40 1\2" wide @ the bottom, 48" wide @ the top, 6ft long, 658lbs empty, 902lbs loaded, 24" chassis ground clearance, 14" axle ground clearance.
I wish you the best in your search & what you come up with.