I lost it. Must have had it too loose and it came off... Until I find a replacement, is there anything I can use as a temp fix?  It still seems to cast fine...

When you order the new cap be sure and order all the spacers as well. Not much you can do in the mean time. I wouldn't fish it too hard with the spool slopping back and forth.

Yep don't use it without, it is a needed part and not just there because it's handy. Most factory replacement OEM caps come as a complete unit, but be sure and ask when you order it.  

Get to the service dept for your reel and order the parts.

I usually double my order when I order parts. I feel if it can happen once it can happen again. I did this order once on a quantum Bill Dance bait caster and I ordered the assembly kit and received the kit and seperate extra parts. Just for ha, ha' s I ordered a out board ball bearing to replace the bushing bearing on the spool and it went right in. I measured it from a newer reel. Just a little upgrade for an older model New reel I purchased at a discount place.

Sometimes I wish the reel manufacturers would offer a small parts kit with springs, gears and parts they know will wear out if they do any life testing on there reels in there R&D lab during there development.

At my last engineering job we life tested everything. We designed equipment to last with regular standard maintence.