So here is your solution. Make a Rita's Rump Cover, but make the back an open pocket like a RRP (if you don't use elastic in the back you probably won't even have to unstuff your pocket when you wash as the insert should wiggle out if the opening is wide enough) and use microfleece/suedecloth or any cotton fabric (flannel/cotton fleece/cotton interlock/sherpa/cotton knit/etc.) as the inner and PUL (waterproof) or fleece (water resistant) as the outer fabric.
If you want elastic in the back, then cut a 1 inch flap of fabric on the outer fabric, just outside of the back elastic points. Before you pin the outer to the inner to sew it together, 3-step zigzag in your elastic on the outer wrong side on your elastic points, just below the flap you cut out. Then turn the flap over to cover the elastic and sew the edges down with a straight stitch (you are making an elastic casing). Then between the elastic points on your inner fabric, don't put in elastic, but instead turn down the fabric approximately 1/2 inch or so and then zigzag the edges down. Pin the diaper, wrong sides together, and starting on on edge of the back pocket, sew all around the diaper, stopping at the other side of the pocket. Turn through the pocket and topstitch (if wanted).
If you don't want back elastic, then just turn both fabrics to the wrong side about 1/2 inch and zigzag the edges, pin, and sew them wrong sides together.
Leg Elastic
If you use a poly inner (microfleece or suedecloth) you can make the elastic roll in or out in the legs, do casings, or even do ruffles. It does not matter since the poly inner should not wick moisture out.
If you use cotton as the inner you need the inner cotton fabric to roll in so that wetness will not wick out. To do this, cut the outer fabric (fleece or PUL) about 1/2 inch wider between the leg elastic points on both sides (curve it gradually back to the normal line where the elastic points are). This makes the outer fabric a bit bigger on the sides.
When you pin the outer to the inner, just line everything up. Your outer will be a little puffy, and this extra fabric will allow the outer to roll in slightly when the diaper is on, preventing wicking. When you sew in your elastic, you will want to sew it in the seam allowance of the outer fabric, before turning. This will help the outer to roll in. You don't want any cotton inner peeking out anywhere when the diaper is on, so you will probably also want to topstitch the diaper (with poly thread if possible) and as you are topstitching, make sure that the outer fabric everywhere (including the wings) rolls under a bit so no inner cotton fabric is exposed to the outside. This is easier than it sounds and very important to have a diaper that won't make baby's clothing wet.
The easiest AIO (All In One) pocket diaper adaption of the Rita's Rump Cover is use thick faux or real minkee or regular fleece for the outer and microfleece or suedecloth for the inner and stitch all around, cut an opening (3-4 inches) in the inner from left to right either in the front or back to turn the diaper, and use this opening to stuff your pocket once turned. Poly knit fabrics like microfleece do not fray, so no need to finish the cut. You only need a few inches since microfleece is stretchy and makes the diaper super easy to stuff. You can also do this with cotton or cotton/poly sherpa or cotton knit fabrics like velour since they do not fray either. However, you always need to finish the edges of flannel or other woven fabrics.
The Rita's Rump Cover adapted this way makes a really trim pocket. This won't fit tiny babies, but it should fit a baby who is a few months old, and my 3 year old can also fit in one as long as the outer and inner fabrics are stretchy (fleece outer/knit poly or cotton inner). You can also use this as a cover with trifolded PFs, so it is an AI2 (All In 2), a pocket or cover.
Any questions/suggestions please leave a comment right here, and I will try to answer it!