Alrighty... almost done with work. Got the BOM (bill of materials) figured out.*
Description of the finished item:
Adapter that will take a 50A service and convert it to two services- (1) 30A 120V and (1) 20A 120V. Both services will be overcurrent protected with the appropriate breaker (30A or 20A) and with lightning/surge suppression on each service. Breakers, suppressors and wiring to be enclosed in a IP-66 certified weatherproof box. Components alone add 37 oz to adapter weight, I am allowing another 8+ oz for connectors, fittings, etc. Targeting 3 pounds (48 oz) additional weight to the adapter (weight 2 pounds, or 32oz). This makes for an entire package weight of 5 pounds (60oz) or less.
I am unable to find specs (yet...) for what a 50A outlet will support for weight. Field testing on our 50A air compressor cord, I was able to put 10 pounds of downward force (VERY conservative estimate, but not measured) on the cable with no signs of plug movement and/or cord distress.
Cost of parts will be ~$200, with the adapter being the most expensive part ($60). The adapter might be superseded by component parts making up the same assembly.
The only thing missing to have this perfectly 'legal' at this point is GFCI protection for the 20A service. This could be provided by not using the adapter, make my own cords and use one of these with an end cut off for the 15/20 Amp plug.-
*LOL...

I dont even make it to the end of the post before changing ideas... but it is firming up, and looking like a good project. It does add ~3 pounds to the adapter, more if I use the GFCI, but the entire setup should weigh
less than a six pack of beer. And provide a longer lasting sense of comfort.

Might have to add a few blinky lights... people LOVE LED's..

WOW... 12 glass bottle beers are 9+ pounds a six pack. Did not know.. thank you intermanet!