Inputbox data saved to two cells and workbook saved as template

I need an inputbox which requests user to enter two serial numbers, which will then be stored in two cells and then the workbook saved as a macro enabled template, which they will use from then on. Whenever their serial number changes, somehow they will need to be able to call up the inputbox to update the cells. Any help is appreciated and thanks!

Thanks alansidman. I think I will use option 1. Now my question is, how does this remain in memory for lack of better word, so when the spreadsheet is closed that this remains somewhere for further use? I only want them to have to enter this info only when required. For people familiar with excel I would ask them to save the sheet as a macro enabled template, but the users will not want to do this, so I need to come up with a macro to do it when you close the input pox maybe?

I recorded a macro of me saving a regular workbook as a template and used that in the clickbutton macro. I was getting "Do you want to overwrite existing file" message, but found how to get rid of that on the internet. I think I am slowly figuring this out albeit maybe a little crude, but it will work for me, for now.