I recently ordered three replacement My K-Cups from an eBay seller. They fit our Platinum K-75 unit, but do not make good coffee. Users of them have made them work better by cutting the lip edge from a used K-Cup such as the hot chocolate K-Cup which doesn't have the built-in basket and inserting it into the basket unit.
I was surprised by the size of the My K-Cup unit. It occurred to me that when Keurig designed them, there must have been a reason. The K-75 Keurig coffee maker has 4 oz., 6 oz., 8 oz., 10 oz. and 12 oz. size options. The 8 oz option is good for normal coffee, and the 10 oz is weaker. I checked on-line recommendations for the amount of coffee to use for a 12 oz cup, and it's 2 coffee measures. Thus, it seems that Keurig knew this and designed the My K-Cup to accommodate a 12 oz size, but they probably realized that allowing owners to use their own coffee might significantly impact their sales of K-Cups. They may also have crippled their original My K-Cup by not restricting the flow of water through the unit to that of a typical K-Cup.
With this in mind, I took a prescription bottle cap which is thick plastic and trimmed the edge to about 1.25 inches so that it would drop into one of the My K-Cup holders. I then drilled a hole in the center of the bottle cap identical to the diameter of the typical bottom hole made in a K-Cup when it is used. I inserted a paper filter (cut from a 4 cup Walmart coffee filter) and stuffed it into the My K-Cup holder. I measured the amount of coffee that a standard K-Cup holds, and it's roughly 1.25 to 1.33 coffee measures of coffee. The paper filter holds 2 coffee measures with the bottle cap insert installed. (Note: Not perfect by any means. The K-Cup leaked at the top of the unit even with the filter extending out over the top of the unit with the cap screwed on.)
What's needed? Simply an insert similar to the one that I made with a 7/64ths inch hole drilled in the center. It would likely be better to have a rubber seal around the edge of the insert to allow it to hold more securely since this size is slightly small to seal well. (Note: You can pop it out with a toothpick from the bottom of the My K-Cup unit when you need to clean it.) It would be helpful if someone would have the basket assembly modified/manufactured to be made longer or deeper to hold the 2 coffee measures of coffee. Coffee filters could also be made to fit without using the basket assembly, but the top of the unit would need either a flat washer or a redesign to screw on snugly since the basket's top edge allows the top to seal tightly when it is used, and this prevents leakage.
Update: The premise about the K-Cup unit size may be correct, but, upon further testing, the filter just doesn't work well for me since it overflows around the cap. I've tried other options including better sealing of the cap as others have recommended, but I still encounter overflows. I used just a coffee filter cut to fit the container (no cap with a hole drilled as noted above), a sawed off sealing ring from one of the basket inserts (to seal the screw-on top better to the unit), and a round washer taped to the cap (to seal the pressure nozzle better). This provided the seal, but it blew out the coffee filter and was messy. This is likely why the K-Cups have strong filters adhered to the premade coffee containers which are sold everywhere. I'm guessing that this coffee making method would work best with a longer pre-made reusable plastic-fiber filter basket similar to the short ones now available in the K-Cup units sold on-line. Using a longer filter basket with one of the sealing methods recommended by others on this site should then enable you to make stronger coffee or larger coffee sizes. I wish someone would make a longer reusable filter basket which would enable us to experiment with the sealing options to ascertain a great way to make K-Cup coffee with our own coffee as we would like it!