There's these. I've never used them, mind. https://www.bikester.co.uk/bicycle-equipment/bottle-bottle-cage/bottle-metal/10302.html?gclid=EAIaIQobChMI2pKKzJWh3gIVxLHtCh2t_ADHEAQYBCABEgIqffD_BwE&_cid=21_1_-1_9_261_10302_201607092657_pla&ef_id=Wi0ozQAAAGR9RjWw:20181025083102:s
That'd do the trick to but the King Cage ones would suit the Mercian better. Well, as much as band-on bottle bosses can suit an old frame...
@Mr_Grun started
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There's these. I've never used them, mind.
https://www.bikester.co.uk/bicycle-equipment/bottle-bottle-cage/bottle-metal/10302.html?gclid=EAIaIQobChMI2pKKzJWh3gIVxLHtCh2t_ADHEAQYBCABEgIqffD_BwE&_cid=21_1_-1_9_261_10302_201607092657_pla&ef_id=Wi0ozQAAAGR9RjWw:20181025083102:s