I have a couple of ideas, but wonder how many preloads you plan to keep at a time? Also, is this for carrying afield, or taking to the range, or for storing a bunch at home?
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I think they have plastic boxes for shot gun shells that would work.
I have a bunch of their plastic boxes for BPCR. I shoot a Sharps .45-110 and they were the only one I found who had boxes that would take my loads with 540 grain bullets.
In a pinch, you could also get by with a filing card holder, made to hold 3"x5" or 4"x6" index cards. They'd be held horizontally instead of vertically, but it might suffice, and is dirt cheap.