The bloc loc ruler concept is great, especially for HSTs. To save your pennies, you can improvise your own locking ruler. To make a locking HST ruler, all you need a ruler ruler that is at least as large as the finished squares you will be making. You can make a locking edge with either post it notes or painters tape. Post it notes work well for smaller sizes, where the length of the post it note is longer then the diagonal of the HST - I find about 3 sheets is a good thickness; peel them off the pad as one unit. Painters tape works well for larger sizes - stack up a bunch of strips longer than the length you need until you get the desired thickness. Then put ruler on top of the the stack and use your rotary cutter to cut through the stack and get a really straight cut. Then put the stack of post it notes or painters tape onto your ruler on the appropriate diagonal line.
Locking rulers rely on having the both seam allowance pressed to one side, so that there is a high side and and a low side of the seam. Take your ruler and slide the tape up to the seam on the low side. As the tape it hits the seam, it will lock in place. One great thing about this is that the if the seam has a bit of a bend to it (possibly from imperfect pressing), the tape will straighten out the seam, so that you end up with square with a straight diagonal as you trim. See if the ruler feels level or if it wobbles a bit. If it wobbles, try adjusting the height of the tape/post-it note stack.
I love making my HSTs slightly oversized and then trimming down with my improvised locking ruler, as it gives me perfect squares with the seams running exactly corner to corner. Not everybody loves trimming down HSTs; trying it out with an improvised ruler lets you try it out the locking way very inexpensively.
Here's a good
video showing the bloc lock HST ruler being used.