Thanks everyone for the comments so far! I like the look of the quadrums, I think it's definitely worth trying one, thanks PWA! Based on craigy's comment it seems like this could be the solution I need.
 
	Andy Jack is actually where I bought my capsules and I found the 19mm hard capsules too small for sixpences, which is how I ended up with 19.5mm. But I haven't tried the foam ones, which might have a bit more give, making 19mm the best option.