Hi, I'm just building my first scratch build cars, a 60 foot Gondola and a 60 foot box car. I wanted some modern looking wagons so I bought two 'HO' cars from my local shop here in the UK and with a set of digital calipers times everything by 3.
I used 8mm perspex to form the frame to give the car strength and so far so good, the measurements were 610mm by 98mm so that when you add the sides using 2mm styrene and then the ribbing it works out at about 110mm which matches the Aristo 53' Evans car.
I drew out an actual size drawing first to plan where the ribs should go and then started from there. I'm using 6.3mm OD brass tube (27mm in length) glued into the perspex to for the truck king pin as this is a perfect fit for the USA trains modern truck. An M5 35mm machine screw soldered in forms the fixing that secures the truck to the frame with a 9mm spacer. So far I'm pleased with how far I've got...... and very surprised I've managed what I've achieved with my clumsy fingers!