New hobbies/projects always get in the way Paul; I've barely touched a kart in months, too busy with other projects, like this one...
I've been working on the frame to hold the main catcher unit the last day or two. While planning, I had to consider size vs weight; too small & it's not viable, too big & it'll weigh too much to work efficiently...
After measuring a few push mower catchers, I deduces that the average size is around 40-50 litres, depending on make & model, with measurements around 500mm long, 350mm wide & a height between 180mm & 250mm.
I knew it had to be bigger than that. I finally decided on a size of 600x300x350mm which would give me a size of around 65 litres.
So, maths lesson over, the first thing I had to do was create the support brackets. Using an adjustable square, I determined the angle of the rear face of the chassis, transferred the angle to a length of 1" x 3/16" angle I had laying around, cut, weld, check & repeat to create two brackets that will protrude straight out the back of the mower.
Then I had to plot the bolt holes in both the brackets & the chassis. I had one hole in the chassis to work off so, I started there. After some careful maths, I had all the holes marked &, after drilling, they all lined up perfectly. Note: I had to drill the holes in the brackets first!