I've had lots of fail with tape/old valves (hard to get the often-lumpy bottom rubbery bit of the valve to seal at the hole without drilling the inside of the rim), but plenty of success turning a 26" tube in to a 29er run strip, complete with valve.Haven't tried the rubber strips, just ghetto Gorilla tape, valves cut from old road tubes, and Stan's.
Cutting the tube to the perfect sidewall-to-sidewall width is a painstaking process, but turns out really bombproof. If you get the final product the perfect width, it creates a really good, solid seal with the bead of the tyre the whole way around. You just need to put 1 layer of that non-stretch filament-type sticky tape around the very inside of the rim (before you insert your custom rim strip) to stop any chance of the spoke holes from rubbing at the rim strip and you're good to go.
Blows up with a floor pump without even using sealant (this snaps the beads in to place) - just add a few soap suds to the beads first before mounting the tyre. Then you can add sealant through the removable-core valve (based on your wise decision to cut up a tube with a removable core), do the shake-it-all-around-hokey-pokey-dance for a bit and go riding with pretty much no worries.
If you haven't tried this, give it a go. Set aside about an hour for making the rim strips. Start by cutting them all the way around, to make 1 flat, wide strip with the valve perfectly in the middle. Then mount it perfectly centred on the rim, as this will tell you how much overhang you need to cut off each side. Mark this with a pen (blue pen shows up as an easily visible line on a black rubber tube, surprisingly) and start cutting on the wider (the outside) side of the line - you don't want the strip to end up too narrow and can always trim more off if it ends up slightly too wide.
That's the only hard part. Everything else is easy.