You can get the cradles here
18650 Battery holders
These will allow you to screw the holder to the roof of the battery chamber as they have screw mount points between the cells as opposed to some of the others that have the mounting holes under the cells which may rub against the cells and cause issues. Even good quality double sided tape would suffice.
I just love double sided tape, there are so many different types that all have their places in the world like the 3M stuff I use to repair iPhones and the spongy stuff for ESC mounting and the gummy stuff that just won't let go etc. I'd use the green backed spongy stuff, and try and cover about 50% of the battery carrier, or just use screws and some tape.
Any 20AWG or 18AWG wire is more than sufficient. Remember we only draw about 4 Amps going full tilt boogie with a standard tank. Try and get high core count silicone as it is so much more durable and flexible and for the few bucks it's probably a good move. If you're like me and broke most of the time then rob an old PC power supply, the leads are long enough and the only real downside is that they are a bit stiff, they are more than up to the task.
I use XT30 connectors, though since I have a lot of deans connectors I do use them sometimes.
You can of course continue with the Tamiya connectors but I see these as redundant as NiCads.
Just bridge one end and you have a 7.4V LiPo in your doggy stalker. Remember to alternate the cells. one side +ve up and the other -ve up. they must add the cells in a row.
A pair of 3.7V LiPos in series. a 5% power upgrade and a more convenient format.
I'll do a set of tutorials for using a 12V 3S1P setup and also for 1hr runtimes or high current usage using a 2S2P 2 series 2 parallel setup where by doubling up we'll get twice the storage and twice the available current.
If you'd like this with pictures etc then just let me know.

Now you don't have to ride a horse to work ...