I’ve got. Yamaha app on my phone that records lean angles. I haven’t used it yet but some of the posts I’ve seen on a Facebook Yamaha page are scary- some people are way more skilled than I am!
Did you try Tow Trust? I reckon this would fit their couplings: https://tow-trust.s3.amazonaws.com/instructions/TCH2.pdf
Couplings: https://www.tow-trust.co.uk/towbars/accessories/height_couplings
Aren’t there rules about belt types in side facing seats? Exmoor recommend lap belts. I seen to remember inertia reels weren’t allowed as they don’t work sideways. I had retractable lap belts (actually still have, just don’t have the seats!).