Sugarfoot,
I know some may like this reply but, I feel (depending on how the nipple is attached) that you should either cut it off or, enlarger it to accept a 1/8" fuse.
A mortar of that size would probably flip backwards as you pull the cord to ignite the percussion cap (This could be a bad thing) :eek: . Also, a mortar should be at a 45 degree angle to the bed which, may make the percussion fixture awkwards but not difficult.
As, for the bed for the mortar. Since, the tube is of confederate style I feel that you should just make a solid flat confederate stlye bed. I'm sure someone will post a photo. However, the bottom line is which do you like best. If, you like the sled style then stick with that.
The tube looks like a nice little mortar so, I wish you the best of luck, be careful and enjoy launching your golf balls.
Keep us informed on your progress.