canklecat

I've used Slime for tubes for that very purpose. You can pour it into a smaller bottle with a plastic nozzle and precut it if you have a tight fitting cap, or cut it to suit the valve later.

Just be sure your tube has a removable valve core and you have a good tool. For example, my Topeak RaceRocket HP and HPX claim to have valve core tools, but they're crap -- just soft plastic that won't handle a tight fitting core. My Blackburn Core Slim includes a metal valve core tool that's actually useful. I used it last night to snug down a core that unscrewed into my Topeak threaded chuck. I prefer threaded chucks for mini pumps, but you do need to check the cores on new tubes to be sure they're snug.

The one time I Slimed a tube rather than patching was over a year ago, before I got a Kool Stop bead jack. I'd just gotten a new set of Continental Ultra Sport II, which were just as tight fitting as other users reported/complained. I tried to seat the bead with tire levers and managed to pinch a hole in a new tube. I was too annoyed to change it because it was so difficult. So I Slimed the tube and ordered a bead jack that day. The Slimed tube held air for a couple of months before finally leaking all at once. Made a huge mess inside the rim. I tossed the tube because it's a huge PITA to clean up a tube so it'll hold an adhesive patch.

I'd also used puncture resistant tubes pre-filled with sealant, but had the same mess when a puncture finally occurred. The goo only slowed the puncture to a slow leak and made a huge mess.

After that I switched to Lezyne self-sticking patch kits. Got one from Mellow Johnny's to test and it worked fine. The whole patch kit -- several paper-thin patches, scuffer and tire boot printed with instructions -- all fit in a plastic envelope only slightly thicker than a credit card and half the size. I keep 'em in every saddle bag now. The Lezyne patch kits are no longer available locally, but I refill the same envelopes with MSW self-stick patches that work even better and are only slightly thicker. These are perfect for patching ultra-snug fitting road bike rims, such as using the thicker Conti Race 28 tubes in 700x23 tires on rims originally intended for 700x20 or narrower tires. Thick glued patches can be a PITA in those situations, but the thin self-sticking patches are perfect.

Get a bead jack. No more pinched tubes from using levers. Kool Stop is probably the best, but not the most compact. I just stuff it in my jersey middle pocket. Doesn't weight much, just has a long handle for leverage. It's actually functional with a shortened handle, so some folks saw it off. And when I strap a spare folding tire onto my larger saddle bag for long rides where I'm beyond Uber/Lyft range, I strap the bead jack under the saddle bag with the spare tire. I've used it twice on my own bike and another two times to assist other folks who were minimalist types -- no spare CO2, no pump, no patch kit, just one tube that they pinched trying to use their one tire lever to horse the tire over the rim. Whatever time and weight they thought they were saving was all expended in one flat, needing help from a passerby.