I don't know what the commercial market builds bumpers to, but this is what I build mine to. Normally if I know that the skin is not for abrasive use I would skin the faces with 7ga (3/16"). If it will be for abrasive use then 1/4". The outer wings (outside of the frame) need bracing to tie to the frame in case of an impact. If a winch is going to be utilized then the interior of the frames need to be a bare minimum of 1/4", for a winch of 4 tons, 3/8" up to 7.5 tons, 1/2" for anything above 7.5 tons. If the rear of the wings is being boxed in then the braces are not a requirement, but are helpful.
If your looking to make it pretty and not really worring about 'using' it, trying to keep weight down, then 1/8" will work, 1/16" will work, but take a carpenters hammer and swing on 1/16', or 1/8" you'll leave a dent.
Boxing of material can do wonders for structural integrity, but for it to survive a hard impact it needs to be built with quality materials. Both the correct size of plate or sheet and the correct placement. Of course then the welding needs to be up to specs for the job as well.
I have been building equipment for heavy commercial vehicles for the past 10 years, since I got out of the service, from ground up fabrication of car carriers (commercial 13-15 car carrier), and now presently I do a lot of work on heavy fire equipment.
Don't overlook the companies that build equipment for our trucks, there are a lot of cool looking designs out there, but there are a lot of junk designs out there too. Try to keep in mind what you want out of a bumper and what will it's job be for you. Once you know what you want out of a bumper, then it's time for you to build it, but if you don't know what you want as an end product then your going to spend a good amount of time staring at some steel.
The info that I put in the first paragraph is info an old timer told me when I first started welding, actually it's on the 4th page of my first welding notebook. I wrote everything that old timer told me down. Yes I still carry a notebook with me today, I would guess I'm on my 8th or 9th notebook today. I have to guess since I went digital a few years ago. If I would start a new bumper Monday for a new winch bumper build this is the same specs that I would use.
For the record, for all of the time I have been building specialized equipment I have had a handful of problems, I have had in doing this for a decade 3 pieces of equipment that 'come back' after being delivered to the end user. My 3 come backs were; 2 had hydraulic issues (leaks) and 1 was an electrical problem (bad factory alternator).
There are only a few other things that I would like to pass on to you and they are, take your time and have fun with it.
