If you look at the ladder track you could have the open side as the right hand side of the barrel jacket and just weld some bits over at the 2 ends, you would obviously have to cut all the long holes with this option though but they would be the right size.
The unistrut comes with different size holes so try and find the largest hole size.
It may be possible to weld one length of ladder track to the other with a 90 degree twist so the receiver has an open bottom. 
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Ok I had my mate over and fed him well, and then hit him with the plans, luckily he liked the idea, so it's all steam ahead. I went and brought the 42 mm unistrut from CEF today, and my mate recons he can flare out the ends enough to fit box section. He's taken the plan that zbear gave the link to and he's going to speak to the guys in the CAD room and see what they come up with. I also went to a salvage yard and brought for a fiver, the end of a pew, made in oak reputed to be 150 years old, that I'm going to attempt to make the stock out of!
This is getting so tempting to have a go myself 
But I know I shouldn't as I have so many other things to do 
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Go on you know you want to, suits you sir 
Another friends dad has offered to mill out a flash hider so that's sorted and I'm going to chat to one of our range members, next week and see if it's possible to make the cocking handle crank back and then spring back in place with a satisfying slap! The only thing I haven't sussed out yet is the magazine or the feed issues.
For the butt and some other parts you could try here http://www.zib-militaria.de/epages/6143 ... 009&Page=2 they're the people who sell the replica's that sgt heide converted
That assault drum mag looks like it's worth sending off for, I may be able to get the feed from a box mag like the A&K minimi to work inside that.
For the assault drum you're better off going here http://www.helstongunsmiths.com/fr_shop-1.html look under accessories they're charging £25 + £3.50 P&P 
Thanks Pete also the plus point with them is that they are Uk based so there's no customs issues. I'm going to mark out the timber later this afternoon and find out exactly how hard old oak is going to be to cut
Shall we start a new thread for "Metal MG42 fabrication" ?
HERE
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For those that fancy plastic I spotted this it is a 10/22 dress up kit for $120.
As importing goes
http://www.amazon.com/Ruger%C2%AE-10-22 ... B0007MTZJS
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I think those ones have been discussed before and they're only about 2/3 scale 
yeah they're mini-me sized 
Yes I used to be on there but my account went when they changed the site.
There was a guy who made one on there when I was on probably the same guy who drew up the diagrams on there.
EDIT nope not that site I was on BBC forum Blaster Builders Club.
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Sly,
B&Q and Wickes tend to be more for the DiYer, try a building suppliers like Build center or Travis Perkins. Failing that do a search for your local building plastics supplier, there's bound to be an independant supplier near you.
Hope this helps 
@ sly, if you're thinking of doing a mg34 have a look at this thread viewtopic.php?f=13&t=2543&hilit=mg34
-for low light situations build an UV led or three into the box mag to charge tracer bb's.
That will look ferkin awesome, I like that idea. A glowing stream of tracer ammo flying at enemy positions in low light conditions, very cool idea
B&Q do a muti surface clear primer called ESP. Stands for Easy Surface Prep and may be worth giving a try. I've used it to prime melamine before.