A 'closed' skirt can be fed from anywhere with cushion air - simplest option is just a hole/slot someplace in the bag that won't catch or get shut (holes needs to be larger than the drain 'leak').  That will produce a 1:1 bag/cushion pressure.  If you want the bag firmer then feed it using a simple small splitter/divider near the lift fan tips area (closer to the fan will increase pressure).    Note that the inner bag shape changes with pressure (1:1 is a flat surface) so careful tailoring will be needed  - high pressure bags also wear quite badly.

Worth using the performance calculator to avoid any inadvertent design issues?

i'm probably doing a full ring bag, as thats by far the simplest skirt to get me going.
its going to have a partition skirt like a sevtec, the skirt is fed from the back and like a sev there is no plenum. its a closed skirt apart from a drain flap at the back, so no feed through.
big hole at the back to chuck air into, but what sort of ratio to under to feed the bag?
Looks like a sev puts about 1/3 into the bag and 2/3 under the craft id like a firm skirt. who doesn't!
i suppose the air can only go where it goes, and it will equalize inside the feed box at the back anyway.
whats your thoughts?