IMO A good choice would be a JK Aerotech T-52. It flys fairly slow, is nearly indestructable, and can pack around the sub-c cells from the RC trucks.

IMO A better choice is a DuskStik from Mountain Models
It's easily built, flys at a walking pace, and is surprisingly tough for a balsa plane, but you won't be able to use the sub-c packs with it.

One of the problems you're going to run into is that most, if not all slow-flyers will not handle wind very well.

I hope this helped, good luck.

Jason