Rule number 25 of HydroWar's website:
http://punkrockcowboy.brinkster.net/hardcore.html
"#25 You have a sign on the back of your Polaris Scrambler that says 'Warning unattended children with Hydroblades will be sold as slaves'."
All jokes aside, you can fashion your own cap with a little clever work. Since the HydroBlade is pressurized reservoir, that limits some of your options if you want to get the job done with as little work and money as possible. In some rare instances, people have found caps around the house that happen to thread onto and even SEAL the reservoir, but that's quite rare to say the least. Here's what I would do, which costs quite a bit of money but is well worth it.
Get a 3/4" PVC ball valve or check valve and a ton of epoxy or epoxy putty. Use the epoxy/putty to secure the valve to the reservoir opening. You'll probably have to make at least some adjustments to get it secured. The problem with ball/check valves is that they take up a lot of space, so you may prefer to build your own valve instead. The advantage to using a ball valve is full control but ball valves cannot be built easily. Check valves have the advantage of being automated; just hook up a pressurized source and it will push water through, though you will more then likely have to push down the piston on the valve manually in order to fill it up. You can also build your own check valve for filling up.
All this can be found at your local home improvement and/or hardware store.
Looking hydroblade(or any 2004 Soakertag gun) tank cap
