There are some solutions you can try that are almost completely non-reversible, but will enable you to fill the blaster without risking spillage while using.
Try finding some pipe fitting pieces that fit as close as possible over the reservoir opening. Epoxy it together and then use the fittings to create your own cap. For the 2007 Secret Strike, you need to drill a tiny hole in the cap to prevent the reservoir from imploding due to the negative outside pressure. (A check valve would work more nicely but costs more, you can design your own though.) For the 2003 Max-D Secret Strike, you just need a solid cap.
Instead of a cap, you can also try using a check valve and/or ball valve. For the 2003 SS, just a ball valve will work. For the 2007 SS, you'll need an air inlet in addition to the ball valve.
Otherwise you're stuck to looking around for a cap. Try the thrift stores if there are any in your area.