battery holder for my trunk?

Trunk battery holder's are pretty common (summitracing.com) but don't forget to look at what is legal in your state too. Many states require that if you have a trunk mounted battery, it must be a sealed type chemical safe container for the battery. Most do not require the sealed casing, but just double check.

I would probably take jl's advice and fabricate one yourself, but you'll want to make some changes vs your uncle's battery holder.

1) Make the casing out of aluminum or mild steel making sure that your structure design will not get near the terminals and potentially shorting out the car. Wood is too fragile (for this application) and will eventually deteriorate especially if the wood gets wet or has chemicals spilled on it.

2) If you are going to carpet, I recommend getting one made for outdoor/marine use. They are treated to resist chemicals and are fire retardant (should the worse happen). Also they will minimize or will not be able to produce static that can possibly ignite the battery fumes (should there be a defect in the battery).

I recommend using grade 8 bolts and nuts to bolt the battery to the floor and minimize damaging the body/frame by using fender washers to help distribute the pressure of the battery if you get in an accident... 15 lb batteries tend to do a lot of impact and chemical damage if they get moving)

Last but certainly not least (for now) HAVE FUN!

Ganbatte ne!