They are designed to fit the SU pads perfectly. They also have inserts to pop in to make the slots less deep.....so you can then fit the staz-on pads and Encore metallics, etc. into the slots. Those pads are the same width as ours just only about halfway deep....if you hold them the correct way.
They have a listing on their website of all the brands and styles of inkpads that fit in the unit. I do remember that the thick style of colorbox pads (such as the full size individual chalk ink) don't fit in any configuration. Those pads are just too tall for each slot. Each individual slot is curved slightly on the edge to help keep the pads from slipping out.
There are excellent graphics and descriptions on this website to help you understand how the system works. They also have traveling options such as covers and straps. I find these are the only system that will hold every type of pad (spectrum hang out the front a bit.....) sold out of the SU catalog. I have been using them for maybe 4 years and have had the ink caddy from SU for half that time.....There is really no comparison. The SU is helpful if you are trying to sell it....but the portaink are solid, sturdy plastic and very well engineered!
I am a fan also because of the excellent service I had when I first purchased. I must have spent twenty minutes on the phone with one of the "engineers" (my day job...
discussing the product and how it came to be....and how it would suit my needs.
Anecdote: They had originally had another product designed altogether and just before launching it....SU introduced the newer ink pad style. One of the engineers wives (probably a demo) called an emergency meeting and they had to go back to drawing board and design a deeper unit that is flexible with the insert pieces!! Why would they even try to sell an inkpad holder that can't fit SU pads?!? So rest assured they work quite well to fit them!