What is the best/easiest way to make the positive battery cable longer? I do not want to cut it.
I need to temporarily put the battery in the rear trunk. I don't want to cut any cables. I plan to get a longer negative cable assembly but that is not possible with the positive cable.

Isn't it already long enough to re-route it if you remove the straps that hold it against the rear bulkhead???

Buy an extension piece, insulate where you bolt it on,you will need a cable that has a post clamp one end and an 8mm eyelet the other,how do you plan the earth the battery negative ?,make sure you fasten the battery down solidly.

If people need such basic instructions, I wonder if they should touch anything connected to power.
/snark

No, hence my question. One of the positive cables is long enough, but the other one is not.

Haha. I like to ask others before I re-invent the wheel.

Thanks. I'm buying a longer cable assembly for the negative.

How would I secure it in the trunk?

One way is to get a 2nd cable and place the new cables eyelet in the old cables terminal clamp. You might have to spread the clamp or use washers to get good spacing so you don't have to squeeze the clamp too far.

Be sure to take this all up, duct tape will work but the glue might need solvent to remove. I've seen fabric "duct " tape in various brands of harnesses where more than 2 wires are spliced together.

EDIT. I see someone else suggested the same thing. You don't need a 8 mm eyelet, the standard 3/8 - 10mm will be fine.

Also, if you ever need to make an eyelet, take a piece of copper tubing and place it over the wires, flatten it then drill a hole through. Instant eyelet.

Not easy without drilling holes...sorry.

Maybe this will help: I'm going to cut out the homemade battery tray that's there now, get the area all grinded and prepped, then drive the car to a welder who is going to install a new, stock battery tray.

Does the battery need to be upright? I could just put it in the old spot, but I'm not sure it will be upright. Welder is about 30 miles away on freeways and suburban streets.

Get a battery that doesn't need to be upright. Meaning not lead-acid. Most will say Optima. I say Odyssey. I use a PC680. Although I'm more likely to explore Lithium Ion next time.