I use a shoe box. JP's sells a box similar to the one in my post above that's supposed to be sized for uncirculated sets but I don't know anything about them.
My 1959 through 2006 sets are in their original envelopes, and those are inside a 4x7 zip lock type clear bag, and the set of them is inside two of those boxes I also got from JP's corner (the ones made for uncirculated sets). The 2007-2019 sets are also in additional boxes of the same type, still sealed in the UPC marked cardboard box.
I remember that, I believe it was in 1999 and was intended to hold the proof sets for the entire state quarter series, all 10 years. (10 sets either clad or the silver sets) Cost was I think $20 for the box. Then in 2007 the President dollars began and the holders changed. All 10 years would no longer fit. Then they added the Territory quarters to the series, so even if they hadn't changed the sizes the entire series still would not have fit. As far as I know they never offered any refunds for the now worthless boxes they sold.