Clean up the end and the sides of the cavity with a sharp instrument to get a good clean base for the new solder to hold. Flux it with a Non corrosive flux and using an electric soldering gun ( I use a common Weller pistol grip type) you can seal it up in nothing more than a few seconds time with no fear of disturbing the ribs at all. No flame to worry about, nice and clean. No need for clamps, heat sinks, wire wraps, etc. Just tin the soldering gun tip well, use the high heat if it has one and add a bit of solder if needed. If the cavity is extra deep (or was open ended), a small tuft of clean steel wool tucked down in there will take up some space and also provide some extra strength to the solder. If it were open ended, it would prevent the solder from running down into the space and instead would fill and plug the end as needed. Clean it off with a file instead of a stone as a stone will just fill with the lead and become useless. Do the final trim with grit paper backed with a file to what ever polish you want. Keep it square!
BTW that solder you bought will work fine, it's just silver bearing solder, same as Brownells Force 44 is. The melting temp is more than OK for the job but most any lead/tin solder will do. The silver bearing solders will stay bright and not turn that dark gray shade like the lead/tin solders do though.
True silver solders have melting points in the 1000* range +/- and are not what you're looking for in this case.
