You might want to look at using LiFe batteries. I have one pack strapped to my Dynatron. MUCH lighter than the lead acid, and one charge seems to last most of the summer. A 4S battery of largish capacity will do for the standard Sullivan. Just select according to the physical size, and how you plan to mount it to the starter. I made a simple tray that hangs below, and has a front and rear end pieces that use the starter bolts for fastening it on the starter. Can be made from 1/8 ply, suitably finished, of course. Velcro the battery to the tray.
I bought a Sullivan 6v electric fuel pump, and connected a 2s 700mah LiFe to it. It too goes a really, really long time between charges. Small enough that I can velcro strap it and the pump to the handle of the fuel jug.
Still use a clip on NiCad/NiMh glow starter - doesn't need a cord to the box. Several versions around. I like the one with the metal case that unscrews like a flashlight, so you can replace the cell if need be.

Of course, that commits you to buying a charger that will charge LiFe. But the batteries charge up in about an hour. And then you will have what you need if you ever decide to dabble in electrics, as most chargers are multi-battery type. No power panel needed, no heavy lead acid that takes forever to charge, no cords to catch in the prop. And more room in the flight box.
BTW - 2S 700 mah LiFe make great flight packs, too for an average sport plane. On the Sig LT-40 I use for training beginners, it can deliver about 13-16 15 min flights before needing a charge. And also charges up in about an hour.