Lots of swearing helps.
I tried to do it all in 3mm sheet, but it's just too thick to form the curves on the side panels, so the mostly flat hatch & frame are done in that. The side panels are thinner, so more easily formed.
The bottom of the frame has a 15ish degree bend in it - this was done over a section of box steel I had with a plastic mallet.
The top of frame has slight curls on the top to either side. I did these by tapping them over a round former (an off cut from the exhaust) until they looked about right when offered up. Then it was just wash, rinse and repeat until they fitted the curve. It was left to run above the tub.
The side panels where the same, but you can form that with your hands. I cut the bottom half to fit the shape of the chassis, and left the top over long. I then gave it a gentle bend in approx the right place with my knee, and then finessed it by hand until it almost fit. The final curve is held by the fixings. Again, these where left over long in height, and length around the curve where it met the boot panel. With the boot panel in place, I could then mark and trim to an exact length.
Once they where all fixed in place, I scribed around the back of them along the top of the tub, took them all off and trimmed it with a jigsaw. I then took a further 5 - 6mm off them all to drop them slightly below the top of the tub.
The hatch is cut directly from the boot panel. A couple of 2 /3mm holes where drilled on the line I wanted, and then it was jig-sawed out. Cutting fluid helps, as it does make the blade cut smoother. Just take your time.
I filed both the frame and the hatch to give a little more clearance.
The lock plate and side 'jambs' that stop the hatch from falling in are 6mm plate I had left over from the driving lamp brackets. These are held on with a couple of M4 counter sunk set screws held in place with nuts so they act as studs. The jambs and lock plate have 8mm holes cut in them to slip over the nuts, and then a washer and a further nut keeps it in place. I did think I was going to have to lock tight them (the screws) in place to stop them rotating once the panel is covered, but there's so much friction there's no need.
As you're only down Bedford way, I suggest you give ASC Metals a ring
A S C Metals Ltd
3 Jackdaw Close
Crow Lane Industrial Estate, Northampton
01604 415036‎
http://maps.google.co.uk/maps?li=d&hl=en&f=d&iwstate1=dir:to&daddr=3+Jackdaw+Close,+Crow+Lane+Industrial+Estate,+Northampton,+NN3+(A+S+C+Metals+Ltd)&geocode=CX0XINV8gyTNFec4HQMdEp3z_w&iwloc=1&dq=asc+metals&cid=13628933248538081620&ei=ovPJSrrxLcrTjAfX8ISZAg
They have been absolutely brilliant - they have loads of offcuts, and they're dirt cheap compared to B&Q or the other DIY sheds. All the Ally I've had for the boot & the driving lamp brackets came to under £40 iirc.