I use to work in a small mining town called Roxby Downs, after one Christmas party two guys flew through a house and came out the other side in a WRX. They ran over a guy sleeping in his bed, miraculously the guy was okay, the bed was made from scaffold poles and he got wrapped up in the mattress. So James you may be okay if a WRX flys through your house. ;-)
so today i made myself a jig to hold the plate on the end as square as i could get it.
i milled a 15mm deep 12mm wide slot into a piece of 2x3 and screwed another piece of 2x3 sideways just overlapping the slot.
i could then clamp this to the end of the 100x50 and put the plate in the slot to hold it squarely in position on the end of the 100x50.
this seemed to work really well as i checked it with a square and seems bob on for what i need.
finally got buzz working again, didn't have the heart to tell him the die grinder weren't plunge in
at least it helps to have a second pair of eyes to check everything is lined up though
so by the end of today I've welded up all the beams that bolt up making the front, back and sides.
need to put it together and check the length in the middle to make up a mid support. i could calculate it but i doubt my cutting and welding is that good to be able to rely on!
still its been fun building it so far.
I'm thinking of putting some 5mm rubber sheet between the bolted plates so it should be able to accommodate any inaccuracy in my welding and obsorb movement that make create squeaks (pet hate of metal frame beds!)
I suspect most metal beds squeak because they are made of tin foil and fixed together with 4 screws. I doubt very much that there will be any give at all in yours once bolted up tight. If anything the rubber could make it worse.