Super Soaker Central

My Ebay CPS 1700 arrived today and it is missing the cap for the water reservoir, I've been trying various caps from bottles around my house but nothing seems to be a good fit. I was hoping someone knew of a product, eg. salad dressing, that had the same sized cap that I could use as a replacement. Also any other ideas are appreciated.

The most common idea that's been tossed around is using a rubber cork to plug it up, but a small hole needs to be drilled.

If you want to take it to the next level though, you could epoxy a check valve (making one yourself may work better) or something similar to the quick fill caps that SS uses. A ball valve is another possibility but you still need a small hole or one way valve to let air in or the reservoir will implode.

I suppose, in the end, the check valve is the most ergonomical solution. Custom building it will allow you to make it more compact as sticking it out can get annoying at best and bust things up (during careless use) at worst.

Either I or someone else can get back on more specific ideas and implementations, but if you don't want a permanent solution (or don't want to do all that work), you'd best stick to the rubber cork.

I would have to reccomend a rubber stopper, one that is for test tubes and the like. Then you can cut a hole into the top to let pressure out. Maybe with a schrader valve.

There's always duct tape.

Except duct tape won't seal yet still isn't good at letting air in.

Using a cork or any other covering will result in minor leakage unless you can drill the hole so tiny that the water doesn't go through. Otherwise you'll have to figure out a water-stopping air inlet solution; make one like the caps SS uses or use a check valve or something similar where a rubber seal is spring loaded to keep water in and let air in as well.

Thanks for all the suggestions!
As a temporary solution I took a rubber sprinkler riser, that fits in the hole fairly snuggly, and then covered the opening of the sprinkler riser with duck tape, and then punched a tiny hole in the duck tape. It lets air in, and only lets water barely dribble out when upside down.
Moving on to getting the pump handle repaired.