Ah I see..having a look it looks like your choices are the doc j I linked one or the two you linked. Both of the ones you linked look like they have a rectangle base.
Reading the review of the iridescent one:
The flared base will ensure that it is not going to get lost inside as it is long and thin, (it fits nicely between the cheeks), not circular like on some plugs, so it is comfortable to wear while walking around.
That's an excerpt from my review! lol.
I highly recommend the iridescent plug. It is a joy to wear and it is also slightly squidy, so it has some give, which is very good for an anal toy! It definitely stays in place very well and is a good girth. This use to be my preferred plug of choice to warm up with as I consider myself fairly experienced with anal play. I don't use it so much anymore, simply because I prefer something a bit bigger hehe