you could really carry some corpses if you had that 90% weight reduction for only 15 pounds
Or haul around a bunch of powder kegs, which would be a lot of fun to be able to do....
Basically, anything that has a large weight meant to prohibit the amount you can carry with you would cease to matter if you could get a 90% weight reduction container. It would have to weigh a lot more than 15 pounds to balance it out in my opinion-I think an "ore bag" made only for ore use would be better. However I also think that just putting your ore in magic bags and giving it to your ox works fine as it is, at least in terms of weight reduction. Of course, ox won't go with you in to the Dvergheim mines which is just really really dumb and sort of necesitates this kind of item for people who want to mine iron and aren't crazy strong themselves.
I think that the reason for needing an ore bag has less to do with needing the weight reduction aspect (magic bags and oxen already exist) and more to allow you the ability to sort, store and put ore on a smelter with the same ease as you can herbs in herb pouches and reagents in alchemy flasks.
A 2x4 bag/pouch looking thing that only stores 1 type of ore and offers no weight reduction is my advice, essentially exactly the same thing as an herbalist bag or alchemy flask but wit ha 2x4 icon instead. You can put these ore bags in your magic bags, put them in storage to hang on to some ore (or ingots-it should also hold ingots) until a later date, or use their unique power to empty them on a smelter/anvil.