Don't worry too much about home brewing your own slider frame, Mick, a shitload of Hagon/ Elstar/ Godden copies were home built in Australia in the '60s and '70s, and most of them were as good as the originals.
The Bully slider that Freaky has was built in Aus from measurements obtained from the UK.
Don Stafford built Elstar copies called Alstars and they were better, IMHO, than the originals, taper roller steering head bearings etc.
I preferred the Elstar frame from the Hagon , for a few reasons, one was the main frame tube finished at the bottom of the rear engine plates , ie the swing arm spindle passed through the frame itself instead of finishing at the top of the rear plates a'la Hagon Sandracer plus the fork sliders were the same diameter all the way through the top fork bridge instead of reducing to a 3/8th or so loooong stud.
I built my first slider frame, around a 350 Ariel motor and gearbox, in 1970 Bro Jim , jimg1au on the forum, still has it
.
Good luck with the project
Foss