for the cap you ought to be able find similar sized PVC
caps- hopefully something small enough to just ram on.
If you could get your hands on some kind of solid plastic cylinder you could probably carve it into a wedge shape and silicone it in there. That'd probably be the best solution, but I'm not to sure where to get the plastic, haha.
Heck, if you just took a soda bottle cap and siliconed it to the end it'd probably hold. As long as you drill enough holes to keep the pressure down (which you'll want to do anyway to save your pump).
As for the # of holes, that'll just be trial and error. depends on how strong you need the current.
Hope this helps, but I have the feeling you'll have figured this out by yourself by now, probably better then what I came up with.
10 gallon 2.6 WPG
3 Dwarf Puffers, 3 Ghost Shrimp, 3 Otos
Planted with Java Fern, C. retrospiralis, Java moss, chistmas moss, L. repens, R. rotundifolia, Wisteria, C. wendtii, C. lucen, S. subulata, M. heterophyllum, C. carolina