Fremen stillsuits are a complex construction. It makes sense; nothing about living on Arrakis should be easy. However, since you don’t actually have to recycle your body’s water, you can get away with making a simpler costume. Instructables user canida came up with a great trick to replicate a stillsuit: use an inside out wetsuit! She also has instructions on making a Jubba cloak.
Black wetsuits look enough like stillsuits to pass unless you’re doing serious cosplay. Note that many colorful wetsuits are all-black on the inside – just turn them inside out. Equip your fremen with a crysknife, maker hooks attached to paracord, and maybe a thumper. A bit of black wire run from the ear to the nose makes it clear you’re serious about reclaiming moisture.
If possible, add a jubba cloak. I made black cloaks for my kids the easy way. Measure shoulder-to-floor distance (drop) and shoulder width. Cut a rectangle of fabric to 1x drop by 2-3x shoulder width, depending on how swirly you want the cape. Measure head length (forehead to crown) and height (neck/shoulder join to top of head), and cut another rectangle that’s 1.5x length x 2.5x height. Fold it in half, and sew a smooth curve for the back of the head. (Best to have the target head around to test against for this step.) Sew a running stitch along the neckline of the cloak and the hood, gather to the same length, then sew them together. Cover and reinforce the join with another strip of black cloth.
Read more at Instructables.