Hi Keith and welcome to the forum. Yes, to say Richard is quite accomplished.. would be a bit of an understatement.
No help around probably cos people think its so easy.
So Bass straps: 2 DIY ways: buy the strap bare, strip of your screw mech (adjuster) and attach it to new strap at the right length attach the other end inside your box.. eh voila as they say.
Or buy the strap ready made (may need to cut the bare end for length adjustment).
In the UK Charlie Marshall stocks both ..
http://www.cgmmusical.co.uk/CGM_Musical_Services/Bass_Straps.html and is really helpful
- maybe worth saying youve not done it before in case he offers additional help Ive not thought of.
So off with the bass plate, undo your adjuster from the knurled nut, measure the overall leather length; decide if you have the tools/capability/time/inclination to transfer your bits across to a bare strap, and fix the other end to the inside... Or not.
Im sure Charlie will have the correct thread for a Hohner, but maybe best to ask, should you decide to go for it pre assembled.
Or hand over to your local repairer..