You might get some ideas from this one: Skid Steer Dual Hitch Receiver | Eterra
I built it for very precise backing of a couple of trailers and for setting implements into my little shop.
Correct. Backhoe... and I use it a lot, so much that I've never removed it since I've owned it, and I can't convince myself to get a 3pt chipper knowing I'll need the backhoe at that same time while doing tree work :/
Scratch, and overkill and this will likely be a small 'boom' for the receiver(s), and I'll build another in the future on a dedicated quick attach plate... I'm thinking for 5-8' to move stuff <1000lbs that I need more reach... like getting a pig out of fenced in area w\out having to remove hog panelsOk, with 48" forks I can see the desire to shrink it down a bit, and if you have trailers with tongue weights that you're not comfortable having on the end of the forks.. yeah those are heavy. Back to your original plan.
I don't have much to add to the actual 'receiver' part of the equation, it should be a fairly simple build. I think building the 'boom' you mentioned is probably where you want to put more brainpower to make sure you never have a failure. I think a boom that engages two 2" receivers on on top of each other (probably 8"+ apart?) should be strong enough for most things. Do you already have a piece from something else you could use to make this thing, or will the boom be made from scratch? Engine hoists and truck bed cranes, game lifts etc usually have multiple length settings with clear markings for weight capacity at each setting and are pretty easy to figure out the dimensions of what they're made out of to give you an idea of what's necessary to hit 'retail ready' levels of safety margin.
One thing you might want to do first is figure out how long you want it to be, and then figure out what your loader's lift capacity is at that distance, so that you can build the thing sufficiently strong without OVER building it for the strength you'll actually have at that distance. I would guess at the end of 5 or 6 feet from the pins you'd have no more than 1000lbs and maybe less. If that's the case you might even consider leaving the boom idea off the hitch piece and.. sorry if im gonna sound like a broken record now, but.. put it on your forks in the form of fork extensions. You have 4ft forks which means you could easily get 6ft extensions and put a crossbar bolted between the extensions with your sling/schackle/hooks on that. If you unbolted the crossbar you'd also have fork extensions..
What I'd REALLY like to find is a small hydraulic winch to utilize for lifting things up too... that's something I'll have to keep an eye out for, not wanting to spend much for a nice to have 
then I, keep in mind this is just a guess, would use a plate type mount as others have mentioned. lot of good ideas in that manner mentioned.
I added a receiver to an old New Holland skid steer forks mountCorrect. Backhoe... and I use it a lot, so much that I've never removed it since I've owned it, and I can't convince myself to get a 3pt chipper knowing I'll need the backhoe at that same time while doing tree work :/
Scratch, and overkill and this will likely be a small 'boom' for the receiver(s), and I'll build another in the future on a dedicated quick attach plate... I'm thinking for 5-8' to move stuff <1000lbs that I need more reach... like getting a pig out of fenced in area w\out having to remove hog panels
