I've put together some rough CAD in my spare moments (which sadly are not many) and I've had a really kind offer from a forum member for some time on their CNC setup so my plans are geared towards that process.

The member's stock of mild in 5mm+ (my design request) is 8mm so I guess it'll be… 8mm

First part is the endplate. There are four of these of the same flattened triangle design. Mounting hardware is all M10 and I try to keep the fitting selection to a minimum to make life easy and sometimes you can save a wee %age on larger quantities

Your stepup is a great way for those people who are trying to build their own swing in the garden but it could be so hard for somebody then in that case you can buy own one at https://babyswingclub.com/best-fold-up-baby-bouncer/ which is a baby bouncer and a swing though, its so easy to see baby happy and relax them