I see. Completely different mounting arrangement from the traditional buggy buckets.
If the nordlocks do not completely solve the issue, try thinner, outside toothed lock washers between the bucket flanges and the mounting base flanges. Use stainless if available for the bolt size so they don’t rust and leave a stain down the mount. Assuming you have 3/8 bolts, it would be these from Mcmaster:
https://www.mcmaster.com/98438a031/Rubber washers would also work and not dig into the flanges but you’d need to be careful about tearing them when adjusting unless you go with something like these abrasion resistant aramid fabric/buna-n washers:
https://www.mcmaster.com/93303A106/
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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS
viewtopic.php?f=5&t=13225&p=134742Simple Spring select
viewtopic.php?f=5&t=11815LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12