Its always a good idea to practice and hone ones skills on a hilt that doesn't have a lot of money tied up in it. Like say a MKIII or any particular run saber.
Items like that are basically an aluminum tube and a blank canvas.
For the led/heatsink module, rather than electrical tape, perhaps a few wraps of the aluminum foil duct work tape from a hardware store (or borrow some if you know anyone that does AC work) The aluminum transfers heat to the hilt body better than electrical tape and it will also solve your gap issue.
I like the idea of upgrading to a li-ion cell as well.
Whether you make it replaceable or in hilt recharge, its a good mod to make.
Also, put some thought into how you want to install the cell/holder. Maybe play around with some kind of chassis? Look up madcow (Genesis custom sabers) on Youtube, his long tutorial on upgrading ascend sabers has lots of insight into inexpensive install options.
And, go to shapeways.com and look around, 3D printing is getting more and more economical by the day, and for those without machinery access, it is a good way to get a chassis that is easy to modify and fit to a particular build.
There are items that can be used with an in hilt recharge 18650 and gives you the option to add sound later on when skill/funds/etc. permit.
A lot at once, I know, but just want you to know that we're all here to help each other improve and better ourselves, and the hobby as a whole.
Good luck!