If you're not too keen on going down to the fabric store, buying some rip-stop
nylon, then coming home to cut and sew it, here's a shortcut:
Go to your nearby discount 'warehouse' store (Costco) and buy a golf umbrella
for about $10. It's just the right size (54in. rib tip to rib tip).
An alternative is to buy a personal-size beach umbrella from any drug
store (available during the summer, natch).
Buy some seam tape - at the fabric store. It's used
to re-inforce the edge of a piece of cloth. It folds over the edge,
covering both sides. Then it's sewn in place.
Remove the ribs from the umbrella (of course).
Sew the seam tape all along the edge of the umbrella. At each joint,
where two panels are sewn together,
make a loop out of the seam tape (a place to tie a string to the
umbrella), then continue sewing.
Cut a hole - 4" diameter or so - at the top of the umbrella. Sew seam
tape around the circumference of this hole.
Buy some mason's twine, or any strong (100lb. or so) braided string,
at the hardware store (or kite store, or...).
Cut the string into 4 ft. lengths. Tie one end to each loop on the
umbrella with a bowline knot. Tie a bowline at the other end. Adjust
so that all strings are the same length.
Buy a small quick-link at the hardware store. attach all the string
(shroud line) ends to the quick link. The quick link is nice because
you can quickly untangle your shroud lines by simply removing the quick
link, then putting the strings back on it, in order.
Attach a short (approx. 18") length of stronger line (400lb or so) to
the quick link, with a bowline knot, and use a larger quick link at the
other end to attach this second quick link, however you like, to your harness.
This second quick link is nice because after you land you can remove the
'chute from your harness, fold it up, stuff it back into its pouch, then
attach the quick link to your harness again. Repack becomes a quick
and easy operation.
If you want to try a PDA (Pulled Down Apex) variation:
Run two or three lengths of shroud line across the opening at the
top of your chute.
Tie a length of shroud line to these lines where they cross.
Tie the other end to the small quick line that holds the other
shroud lines.
Adjust the length of this center line so that you get the desired
amount of pull-down.