I'll second Jim on the mini square and their handiness for guitar work. (especially heels)
I have 2 that use regularly. One is 50mm machinist square and the other is a 75x100mm flat square (Hobby Tools Australia).

Jim's comment on scraping vs sanding is valid as well. The Stanley knife blade is a good economical alternative to a set of cabinet scrapers and are very useful for these kinds of jobs. I'm a recent acquirer of cabinet scrapers and will be getting a set of mini scrapers soon to add to my tool kit.

Lastly, if sanding is your only or preferred option, it's completely doable to make a DIY right angle sanding block. It's perfect for doing heel work. If you google it, there are a few common approaches to making them so you can choose which suits you best. I have one I made years ago and have used it a lot.