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You might struggle to get double sided tape to stick to rubber, you'll probably be better off with foam - the sort that sleeping/yoga mats are made of. (closed cell polyethylene foam)
Or, if you were able to make a couple of small moulds, fill them with silicone - the sort of stuff you use round the edge of your bath, and when that has set, bond them in place using the same silicone. I think the silicone one will be the most robust of the three. I've got some spacers from Shimano Sora levers which are surplus to requirement. PM me a postal addy and I'll put them in the post to you - see if you can make them fit.
My partners got drop bar advent x on her new bike, but she has quite small hands. Microshift make a reducer for some of their levers, but not these.
Has anyone made their own spacers to reduce reach for other levers? Thinking of just cutting a bit of foam or rubber and double sided tape it in?