I do not really know any backwoods ways that work well long term. I do know the right way to do this repair.
If you want real cheap back woods work buy a can of spray foam and fill the rocker in. Then after removing the duct tape you will have something that is the right shape. Later on it is a big hassle to remove. This is not good bodywork in any way, shape, or form.
Typically once part of a rocker panel starts to rust a complete replacement is the only long lasting solution.
Every body panel is held on with a lot of spot welds. You can recognize them because they are small dots that are lower than the rest of the panel. You can use a spotweld cutter (probably $20-$30)to cut them out. Often it does not damage the back panel but sometimes it does. You can use a regular drill bit (under $2) the right size and drill through both panels. You can also use tin snips to remove most of the bad metal and then use a grinder to grind the metal over the spotwelds so thin that using a flat blade screwdriver you can pop the metal off.
Be careful with the grinder, if you look at a pretty girl walking by while grinding the sheetmetal you could grind away the back panel also. Pay attention and stop grinding when you see the back panel.
When it comes time to replace the rocker remove all rust on the back panel. Cover it up with paint at a minimum. Using a 95% zinc, 5% epoxy weld through primer (which I could never weld through) is cheap insurance for rust prevention. Painting the inside of the new panel is also a good idea. If you leave any of the old rocker panel in place it is just inviting more rust to appear later on. Make a factory sized hole for a factory sized part. Just be sure the replacement panel is big enough for the job and is not made 1" to short.
If you want to weld the panel on imitate the factory method. Drill several holes in similar locations to what the factory had for spot welds. You can do this on either the new rocker or the backing plate. Once the holes are there you can install the panel.
(unplug the batteries and computer at this time)
Before you begin to put the panel on get several vise grips. Clamp the panel on, shut the door and make sure it is lined up correctly. Now you can clamp down the top of the rocker panel. Now you will find out the reason for the holes you drilled. They should be between 1/8" and 1/4". Take the welder and fill the holes with weld. You may find making circles when filling helps. Try this OFF the truck first on scrap metal. Once you have the technique down put the panel on.
As far as the inside of the truck goes you may end up making a patch panel. This will be mostly getting some thin tin and beating it to fit the shape you need. This is not a critical job. Use a hammer, use some pliers, use whatever you can to get it close. Once it is close push the panel onto the original metal. With it pushed into place (pushing with something if needed) put a bit of a weld bead down. Beat and push the new metal as needed until the hole is covered and welded closed. Continue until the patch is completely installed. The floor is basically flat (ignore the strengthening ribs). You do not need to worry about warpage as the carpet will cover everything anyway. Do use some common sense and do not weld more than about 2" at a time without allowing some cooling to happen. You can probably weld up 18" at once without serious concern about warpage, but pay attention and remember, a little time now can save a lot of trouble later on.