My field start tote box has two 4.5Ah 12V gelcells, which fits where a typical 7Ah 12V brick would go. I rewired the Tower Hobbies power panel to provide terminals for either 12V or 24V. I had used my old Tower HD starter on 24V for many years with no problem - it actually died of a bent shaft (my fault). Post mortum autopsy revealed a pair of brushes still in good serviceable shape. Now I use a new Hobbico HD 180 starter. No problem either.

As for how to engage the prop bolt, you can get prop bolts predrilled for 10-32 thread, if you don't want to do it yourself. B&B sells a very pretty prop nut that fits over a hex bolt head and tightens with a 10-32 set screw.

Alternatively, you can find a long 3/8" bolt, cut off the bolt head, and make a stud out of it. Then you can locktite it to the prob hub and use a regular spinner nut.