Hi Tom,
i wanted to highlight that redfish has two options - the “custom seat” option which is $255 (under the “seats” tab of their menu) and the “presculpted seat blanks” under the “materials, minicell foam” tab of the menu and these range from $75 to $85 depending on size.
the full custom seat includes back rest and side braces so you would not install any of these in the boat. the seat blank option is meant to work with a traditional backband and cheakplates which a CLC 17 LT kit already includes and you may have already installed.
so for a 17lt, i would go with the seat blank…and that is what i have done in builds like that. so its actually a very reasonable cost. i don’t glue them in…but wedge them under the side braces
in the picture below is a redfish seat blank installation on one of my kayaks that has cheek plates (the wooden verticals on either side of the seat) minicell (from the kit) glued to the cheekplates, and backband/strapping/screws that came from the kit.
the seat in this case is wedged under the foam elements on the side that are glued to the cheek plates (you push the seat forward to remove it, and slide it back to lock it in place). i also used some minicell from the kit (i cut it from the seat-shaped piece) to elevate the front of the seat blank…which is an adjustment i made for extended comfort after paddling for a while and realizing i needed some more up-pitch to the seat:
this is another angle on that installation showing the backband attachent to the cheek plate and how the seat is wedged in:
the next picture is a redfish custom seat in another of my boats. you can see there is no cheek plates or backband in this arrangement. if there is any downside to this arrangement, its that you can’t easily ‘repitch’ the seat angle like i did with the blank by adding some material underneath the front:
that said, both of these work very well and have proven very comfortable.
i would also highlight that the seat blank and custom seat have the same basic carved space for your butt…there is no customization there. what is customized in the one-piece unit is that it is precarved to the internal hull measurements you provide. in the seat blank, you have to carve that yourself.
while i have not tried it, guillemont recently began offering seat blanks options as well: Foam Kayak Seat - A Comfortable Kayak Seat for New Builds or Replacement | Guillemot Kayaks that are similar to the redfish seat blank. check out their website at the link i posted.
the main difference i see in the guillemot offering, is the seat they sell is only 16 inches wide, so you would need to glue it in (i wonder if you call them in advance, perhaps they could leave more material on the side?). the redfish seat blank comes with a lot of extra material and is wide enought to span completely between the cheek plates… allowing you to tuck it under your side minicell…so you can easily remove it and clean/dry the boat then reinsert it/wedge it into place…which is my preferred approach.
as mentioned above, if this was my first boat, i would probably go with the redfish seat blank…but read the guillemot site carefully because i think they give a nice overview of the fit considerations that you would need to apply to the redfish seat blank.
when you work with the seat blank you just need to work slowly and carefully to not take off too much material resulting in it being too small…easy to take material off…but no easy way to add it back if you cut too far 
h