RE: Making a one-off custom tank
I've done several, and always used plain old fiberglass. In an impact, the kevlar bursts just as easy as glass does...
I make a one piece blue/pink-foam plug, make sure your corners are rounded. I make it 3/1 to 1" longer than I need, so that it can be cut into two pieces. Cut the plug into two, 1/8-3/16" longer than you want the big part to be. I wrap several layers of masking tape around the cut end of the short piece to make an offset for a flange. I cover the whole thing in SolarTex, which is a really low heat film, then wax and PVA the plugs.
lay up the glass (I use a layer of 2-ounce, then a layer of 6 and another layer of 2 ounce - just because that has worked for me before). When cured overnight, I take a VERY thin spatula (ask your dentist for a CoeSoft mixing spatula) and get the lip open, then put several drops of water in the open area and pop up the sides. If it won't slide off, I use a football filling needle on my compressor, give a squirt and make sure that you catch the plug before it flies across the room. I like to use BVM's fittings.
A trick told to me is to then wash and acetone the inside of the tank, then flow fresh epoxy around the inside to fill pinholes that will happen in the corners (I also do the outside) then trim and assemble the two parts of the tank with Hysol or Aeropoxy. Make sure you pressure/leak test!
This has worked for me, and gives a plug for future use/ screw-ups.
Greg