i prefer snoozer brand seats, i dont like the suspended ones because ive seen the webbing snap and stitching come undone real easy.
the snoozers are expensive but its realy easy to mkae something similar.
option 1:
take 2 milk crates, one goes upside down and one goes ontop right way up, zip tie togehter.
using heavy duty cutters or a small saw cut holes in each side so you can slip the cars seatbelt through the back of the seat and clip it in.
add a foam pad or bed and pretty-fy with a sheet tucked in as a slip cover almost. a deacent coupler wrapped around the seatbelt and clipped to a wide chested harness (ie puppia or road hound)
you can also use those cube shelving units to make a custom sized one (i made mine this way because i wanted a seat big enough for my 2 to have plenty of room while still fit on my front seat (airbag has been disabled!) this is a HEAVY unit though and not for someone who wants somthing that can easily be stored in the trunk of a car lol, though im sure with some creativity there would be a way of doing it to make it collapsable lol)
take each peice, figure out how long you want it and cable tie together the bottom, mine is 1 1/2 cube peices long (about 15 inches) then attatch the sides, again with zip ties, you want the bottom to sit INSIDE the sides so theres a wall around the seat, and remmeber youll be adding a layer of foam (your choice of thickness) so keep that in mind when your placing it on the sides.
once its tied together itll look funky but dont worry...
now as with the milk crates, metal snip to make holes for the seatbelt, the seatbelt holds the entire thign in place in a sudden stop.
now add a peice of thick card and a foam pad in the bottom, i had extra foam so added it to the entire inside seat including front and side walls but the wall padding of it can be acomplished with batting...spray glue (or more cable ties) will keep it in place. drape with a sheet (or if your sewing savvy make a cover for it) t gets seatbelted in, a heavy dty couple clips around the seatbelt and TaDa...
yes it sounds difficult, thats why i included pictures...
but oce you get the idea of what your building, its realy easy...(and kinda fun putting the peices together lol)
(the open part where the foam hits the pack is where i cut out the hole for the seatbelt to fit through, a matching hole is on the other side)
(i covered the whole thing in batting for a finnished more plush look when covered, its not a must.)
(dodger trying ti out)
the finnished product in the car, the cover is removable and the whoel thing cost under $30.00 to make cause i used $1.00 per yard fabrics and got a peice of damaged foam for cheap!