Spent way too long figuring out how I was going to accurately cut up the aluminum for the rails. Do I buy a drop saw, milling machine, hand router, etc. The thing is I didn’t really want to buy any of those tools, so chopped it up with a hacksaw (helped by some 3D printed miter boxes) and got the lengths down to a good tolerance/square with a crappy little disk sander.
It worked really well in the end; both rails were within 0.1mm of each other.
What didn’t go so well was drilling holes where I wanted them to go, but this did improve over time. Tried center punching first, and that was ok’ish but not great. Then moved on to what I believed was the correct approach of using a center drill first then the actual drill bit. This worked better, but I often found the small tip would snap within the part you were drilling. You then get stuck with a bit of HSS drill bit stuck down the hole you want to drill. I destroyed so many bits.
The center drill (left) is to be used for use on lathes, what I should have been using is a spotting drill bit (right) which is also sometimes referred to as a center drill bit 
. Picked up a few cheap countersink and counterbore bits from aliexpress, the power button counterbore was 20mm in diameter which my small drill press handled surprisingly well.
Tapping threads is now my least favorite thing in this build. There are about 40 m4 threads that needed to be done. The little 3D printed part did help keep the tap perpendicular, still sucks.
I did leave some of the tapping till after the frame was welded, but only because I was so over it. Marked up the different parts and dropped it off at a local fab shop that specialised in car intercoolers/catch cans/etc.
Same day service is not what I was expecting, but sure enough I got a call back that same day saying the job was done.
Welds look good, pity they’ll all be covered up.
In hindsight, I wouldn’t go for a welded frame again. Cost, makes it harder to work on, can’t change the width of the board (for wider/slimmer wheels).